[SPARK-21508][DOC] Fix example code provided in Spark Streaming Documentation
[SPARK-21508][DOC] Fix example code provided in Spark Streaming Documentation

## What changes were proposed in this pull request?

JIRA ticket : [SPARK-21508](https://issues.apache.org/jira/projects/SPARK/issues/SPARK-21508)

correcting a mistake in example code provided in Spark Streaming Custom Receivers Documentation
The example code provided in the documentation on 'Spark Streaming Custom Receivers' has an error.
doc link : https://spark.apache.org/docs/latest/streaming-custom-receivers.html

```

// Assuming ssc is the StreamingContext
val customReceiverStream = ssc.receiverStream(new CustomReceiver(host, port))
val words = lines.flatMap(_.split(" "))
...
```

instead of `lines.flatMap(_.split(" "))`
it should be `customReceiverStream.flatMap(_.split(" "))`

## How was this patch tested?
this documentation change is tested manually by jekyll build , running below commands
```
jekyll build
jekyll serve --watch
```
screen-shots provided below
![screenshot1](https://user-images.githubusercontent.com/8828470/28744636-a6de1ac6-7482-11e7-843b-ff84b5855ec0.png)
![screenshot2](https://user-images.githubusercontent.com/8828470/28744637-a6def496-7482-11e7-9512-7f4bbe027c6a.png)

Author: Remis Haroon <Remis.Haroon@insdc01.pwc.com>

Closes #18770 from remisharoon/master.

cloud-fan/spark,zero323/spark,JoshRosen/spark,wzhfy/spark,ahnqirage/spark,spark-test/spark,tengpeng/spark,sahilTakiar/spark,lxsmnv/spark,techaddict/spark,bdrillard/spark,lvdongr/spark,stanzhai/spark,milliman/spark,holdenk/spark,mdespriee/spark,wangyum/spark,yanboliang/spark,pgandhi999/spark,HyukjinKwon/spark,hvanhovell/spark,bravo-zhang/spark,tejasapatil/spark,ueshin/apache-spark,taroplus/spark,saltstar/spark,byakuinss/spark,apache/spark,nlalevee/spark,shaneknapp/spark,pgandhi999/spark,1haodian/spark,WeichenXu123/spark,zzcclp/spark,actuaryzhang/spark,minixalpha/spark,BryanCutler/spark,ddna1021/spark,ptkool/spark,zero323/spark,vinodkc/spark,akopich/spark,kevinyu98/spark,wangyum/spark,ptkool/spark,zuotingbing/spark,chuckchen/spark,BryanCutler/spark,ajaysaini725/spark,ahnqirage/spark,chuckchen/spark,guoxiaolongzte/spark,joseph-torres/spark,hvanhovell/spark,xuanyuanking/spark,WeichenXu123/spark,facaiy/spark,mike0sv/spark,spark-test/spark,akopich/spark,maropu/spark,lvdongr/spark,dongjoon-hyun/spark,aray/spark,mdespriee/spark,gengliangwang/spark,ptkool/spark,someorz/spark,hhbyyh/spark,aray/spark,ioana-delaney/spark,cloud-fan/spark,goldmedal/spark,holdenk/spark,byakuinss/spark,akopich/spark,lvdongr/spark,witgo/spark,michalsenkyr/spark,srowen/spark,maropu/spark,shubhamchopra/spark,WeichenXu123/spark,minixalpha/spark,dbtsai/spark,srowen/spark,ueshin/apache-spark,wzhfy/spark,tejasapatil/spark,holdenk/spark,eyalfa/spark,shaneknapp/spark,xuanyuanking/spark,byakuinss/spark,mahak/spark,sureshthalamati/spark,HyukjinKwon/spark,kevinyu98/spark,UndeadBaneGitHub/spark,ajaysaini725/spark,actuaryzhang/spark,vinodkc/spark,markhamstra/spark,gengliangwang/spark,aosagie/spark,apache/spark,milliman/spark,adrian-ionescu/apache-spark,WindCanDie/spark,saltstar/spark,bravo-zhang/spark,xflin/spark,LantaoJin/spark,aosagie/spark,szhem/spark,mdespriee/spark,michalsenkyr/spark,shaneknapp/spark,shubhamchopra/spark,michalsenkyr/spark,witgo/spark,szhem/spark,brad-kaiser/spark,zuotingbing/spark,techaddict/spark,narahari92/spark,LantaoJin/spark,hvanhovell/spark,guoxiaolongzte/spark,rednaxelafx/apache-spark,mahak/spark,BryanCutler/spark,aosagie/spark,alunarbeach/spark,eyalfa/spark,cloud-fan/spark,nchammas/spark,debugger87/spark,saltstar/spark,zzcclp/spark,saltstar/spark,gengliangwang/spark,wzhfy/spark,minixalpha/spark,darionyaphet/spark,yanboliang/spark,aosagie/spark,esi-mineset/spark,shaneknapp/spark,SHASHANKB/spark,guoxiaolongzte/spark,ddna1021/spark,ioana-delaney/spark,ueshin/apache-spark,jkbradley/spark,cloud-fan/spark,narahari92/spark,hhbyyh/spark,holdenk/spark,shuangshuangwang/spark,ddna1021/spark,taroplus/spark,UndeadBaneGitHub/spark,skonto/spark,mike0sv/spark,wangyum/spark,LantaoJin/spark,someorz/spark,tengpeng/spark,lvdongr/spark,Aegeaner/spark,dbtsai/spark,lvdongr/spark,SHASHANKB/spark,techaddict/spark,alunarbeach/spark,alunarbeach/spark,wangmiao1981/spark,janewangfb/spark,zero323/spark,rekhajoshm/spark,pgandhi999/spark,aray/spark,1haodian/spark,cin/spark,brad-kaiser/spark,yanboliang/spark,rednaxelafx/apache-spark,zhouyejoe/spark,jkbradley/spark,Aegeaner/spark,saltstar/spark,wangmiao1981/spark,markhamstra/spark,skonto/spark,wangmiao1981/spark,nchammas/spark,WeichenXu123/spark,alunarbeach/spark,hhbyyh/spark,ioana-delaney/spark,joseph-torres/spark,vinodkc/spark,adrian-ionescu/apache-spark,esi-mineset/spark,zero323/spark,lxsmnv/spark,bdrillard/spark,kiszk/spark,rikima/spark,dbtsai/spark,dbtsai/spark,joseph-torres/spark,darionyaphet/spark,ajaysaini725/spark,narahari92/spark,sahilTakiar/spark,mike0sv/spark,rikima/spark,shubhamchopra/spark,highfei2011/spark,zero323/spark,tengpeng/spark,rekhajoshm/spark,goldmedal/spark,stanzhai/spark,nlalevee/spark,shuangshuangwang/spark,cin/spark,HyukjinKwon/spark,dongjoon-hyun/spark,vinodkc/spark,cin/spark,nchammas/spark,zhouyejoe/spark,shuangshuangwang/spark,HyukjinKwon/spark,zhouyejoe/spark,zzcclp/spark,lxsmnv/spark,narahari92/spark,yanboliang/spark,zhouyejoe/spark,UndeadBaneGitHub/spark,akopich/spark,rekhajoshm/spark,ericvandenbergfb/spark,dbtsai/spark,apache/spark,maropu/spark,HyukjinKwon/spark,techaddict/spark,tengpeng/spark,adrian-ionescu/apache-spark,mahak/spark,apache/spark,Aegeaner/spark,xuanyuanking/spark,gengliangwang/spark,WindCanDie/spark,ron8hu/spark,shaneknapp/spark,mike0sv/spark,xuanyuanking/spark,wangyum/spark,minixalpha/spark,shuangshuangwang/spark,xflin/spark,gengliangwang/spark,mdespriee/spark,nchammas/spark,ueshin/apache-spark,ConeyLiu/spark,sureshthalamati/spark,pgandhi999/spark,markhamstra/spark,holdenk/spark,cin/spark,apache/spark,mahak/spark,ericvandenbergfb/spark,xflin/spark,stanzhai/spark,byakuinss/spark,apache/spark,ConeyLiu/spark,aosagie/spark,LantaoJin/spark,goldmedal/spark,byakuinss/spark,hhbyyh/spark,highfei2011/spark,adrian-ionescu/apache-spark,spark-test/spark,ericvandenbergfb/spark,taroplus/spark,adrian-ionescu/apache-spark,wangyum/spark,jkbradley/spark,cloud-fan/spark,tejasapatil/spark,rikima/spark,rikima/spark,eyalfa/spark,skonto/spark,eyalfa/spark,1haodian/spark,szhem/spark,caneGuy/spark,WindCanDie/spark,rednaxelafx/apache-spark,nchammas/spark,SHASHANKB/spark,goldmedal/spark,rezasafi/spark,sureshthalamati/spark,wangmiao1981/spark,Aegeaner/spark,Aegeaner/spark,ConeyLiu/spark,sahilTakiar/spark,sureshthalamati/spark,darionyaphet/spark,LantaoJin/spark,LantaoJin/spark,rekhajoshm/spark,kiszk/spark,pgandhi999/spark,ajaysaini725/spark,kiszk/spark,srowen/spark,ahnqirage/spark,markhamstra/spark,esi-mineset/spark,janewangfb/spark,HyukjinKwon/spark,lxsmnv/spark,UndeadBaneGitHub/spark,bravo-zhang/spark,kevinyu98/spark,ericvandenbergfb/spark,ericvandenbergfb/spark,ahnqirage/spark,facaiy/spark,wzhfy/spark,eyalfa/spark,srowen/spark,xflin/spark,JoshRosen/spark,1haodian/spark,narahari92/spark,mike0sv/spark,facaiy/spark,ron8hu/spark,mdespriee/spark,zuotingbing/spark,ericvandenbergfb/spark,someorz/spark,wzhfy/spark,kiszk/spark,shuangshuangwang/spark,janewangfb/spark,kevinyu98/spark,JoshRosen/spark,taroplus/spark,icexelloss/spark,shubhamchopra/spark,mike0sv/spark,spark-test/spark,HyukjinKwon/spark,actuaryzhang/spark,cloud-fan/spark,ron8hu/spark,rezasafi/spark,zhouyejoe/spark,jiangxb1987/spark,adrian-ionescu/apache-spark,debugger87/spark,milliman/spark,stanzhai/spark,rezasafi/spark,byakuinss/spark,mahak/spark,highfei2011/spark,zzcclp/spark,bravo-zhang/spark,aray/spark,markhamstra/spark,ajaysaini725/spark,dongjoon-hyun/spark,jiangxb1987/spark,ddna1021/spark,ron8hu/spark,dongjoon-hyun/spark,debugger87/spark,SHASHANKB/spark,milliman/spark,byakuinss/spark,rezasafi/spark,darionyaphet/spark,rezasafi/spark,ericvandenbergfb/spark,xflin/spark,WeichenXu123/spark,ConeyLiu/spark,icexelloss/spark,skonto/spark,eyalfa/spark,bdrillard/spark,wangmiao1981/spark,esi-mineset/spark,WindCanDie/spark,wangyum/spark,saltstar/spark,rekhajoshm/spark,debugger87/spark,facaiy/spark,vinodkc/spark,ron8hu/spark,xflin/spark,ptkool/spark,tejasapatil/spark,ConeyLiu/spark,spark-test/spark,jiangxb1987/spark,akopich/spark,holdenk/spark,rekhajoshm/spark,joseph-torres/spark,actuaryzhang/spark,sahilTakiar/spark,rezasafi/spark,highfei2011/spark,lxsmnv/spark,rezasafi/spark,witgo/spark,JoshRosen/spark,esi-mineset/spark,akopich/spark,zzcclp/spark,hvanhovell/spark,BryanCutler/spark,bravo-zhang/spark,jkbradley/spark,nlalevee/spark,kevinyu98/spark,ioana-delaney/spark,debugger87/spark,icexelloss/spark,taroplus/spark,adrian-ionescu/apache-spark,WindCanDie/spark,ahnqirage/spark,zero323/spark,1haodian/spark,hhbyyh/spark,goldmedal/spark,UndeadBaneGitHub/spark,witgo/spark,cin/spark,sahilTakiar/spark,ioana-delaney/spark,ron8hu/spark,ddna1021/spark,saltstar/spark,xuanyuanking/spark,wzhfy/spark,joseph-torres/spark,witgo/spark,tejasapatil/spark,sureshthalamati/spark,maropu/spark,milliman/spark,minixalpha/spark,UndeadBaneGitHub/spark,eyalfa/spark,gengliangwang/spark,ioana-delaney/spark,Aegeaner/spark,JoshRosen/spark,darionyaphet/spark,bravo-zhang/spark,markhamstra/spark,1haodian/spark,ConeyLiu/spark,someorz/spark,taroplus/spark,ajaysaini725/spark,chuckchen/spark,joseph-torres/spark,jkbradley/spark,shubhamchopra/spark,LantaoJin/spark,hhbyyh/spark,apache/spark,techaddict/spark,skonto/spark,ddna1021/spark,caneGuy/spark,guoxiaolongzte/spark,highfei2011/spark,zzcclp/spark,minixalpha/spark,milliman/spark,shaneknapp/spark,facaiy/spark,minixalpha/spark,brad-kaiser/spark,nlalevee/spark,kevinyu98/spark,WeichenXu123/spark,shubhamchopra/spark,szhem/spark,matthewfranglen/spark,facaiy/spark,dbtsai/spark,nlalevee/spark,michalsenkyr/spark,hvanhovell/spark,xflin/spark,janewangfb/spark,icexelloss/spark,szhem/spark,gengliangwang/spark,narahari92/spark,markhamstra/spark,narahari92/spark,actuaryzhang/spark,hvanhovell/spark,bdrillard/spark,JoshRosen/spark,guoxiaolongzte/spark,esi-mineset/spark,mdespriee/spark,pgandhi999/spark,kiszk/spark,JoshRosen/spark,sureshthalamati/spark,sureshthalamati/spark,zuotingbing/spark,brad-kaiser/spark,cin/spark,zuotingbing/spark,aosagie/spark,chuckchen/spark,kiszk/spark,bdrillard/spark,srowen/spark,lxsmnv/spark,guoxiaolongzte/spark,debugger87/spark,ueshin/apache-spark,kevinyu98/spark,michalsenkyr/spark,nchammas/spark,actuaryzhang/spark,dongjoon-hyun/spark,stanzhai/spark,UndeadBaneGitHub/spark,BryanCutler/spark,janewangfb/spark,pgandhi999/spark,wangyum/spark,shuangshuangwang/spark,dbtsai/spark,Aegeaner/spark,dongjoon-hyun/spark,ueshin/apache-spark,sahilTakiar/spark,zhouyejoe/spark,cloud-fan/spark,akopich/spark,holdenk/spark,icexelloss/spark,mahak/spark,shuangshuangwang/spark,zuotingbing/spark,caneGuy/spark,rednaxelafx/apache-spark,nchammas/spark,debugger87/spark,ptkool/spark,wzhfy/spark,zzcclp/spark,shaneknapp/spark,jiangxb1987/spark,ajaysaini725/spark,bdrillard/spark,shubhamchopra/spark,jiangxb1987/spark,highfei2011/spark,WeichenXu123/spark,szhem/spark,highfei2011/spark,caneGuy/spark,goldmedal/spark,aray/spark,janewangfb/spark,janewangfb/spark,rikima/spark,skonto/spark,yanboliang/spark,darionyaphet/spark,rikima/spark,ioana-delaney/spark,witgo/spark,vinodkc/spark,kiszk/spark,rednaxelafx/apache-spark,ConeyLiu/spark,jiangxb1987/spark,spark-test/spa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