[SPARK-24134][DOCS] A missing full-stop in doc "Tuning Spark".
[SPARK-24134][DOCS] A missing full-stop in doc "Tuning Spark".

## What changes were proposed in this pull request?

In the document [Tuning Spark -> Determining Memory Consumption](https://spark.apache.org/docs/latest/tuning.html#determining-memory-consumption), a full stop was missing in the second paragraph.

It's `...use SizeEstimator’s estimate method This is useful for experimenting...`, while there is supposed to be a full stop before `This`.

Screenshot showing before change is attached below.
<img width="1033" alt="screen shot 2018-05-01 at 5 22 32 pm" src="https://user-images.githubusercontent.com/11539188/39468206-778e3d8a-4d64-11e8-8a92-38464952b54b.png">

## How was this patch tested?

This is a simple change in doc. Only one full stop was added in plain text.

Please review http://spark.apache.org/contributing.html before opening a pull request.

Author: Xiaodong <11539188+XD-DENG@users.noreply.github.com>

Closes #21205 from XD-DENG/patch-1.

chuckchen/spark,witgo/spark,ueshin/apache-spark,pgandhi999/spark,nchammas/spark,hhbyyh/spark,cloud-fan/spark,witgo/spark,goldmedal/spark,zuotingbing/spark,matthewfranglen/spark,aosagie/spark,rekhajoshm/spark,mdespriee/spark,dongjoon-hyun/spark,pgandhi999/spark,JoshRosen/spark,mdespriee/spark,debugger87/spark,maropu/spark,apache/spark,bravo-zhang/spark,lvdongr/spark,LantaoJin/spark,caneGuy/spark,bdrillard/spark,WindCanDie/spark,actuaryzhang/spark,lxsmnv/spark,michalsenkyr/spark,rekhajoshm/spark,highfei2011/spark,taroplus/spark,michalsenkyr/spark,witgo/spark,milliman/spark,spark-test/spark,caneGuy/spark,nchammas/spark,kiszk/spark,zhouyejoe/spark,dbtsai/spark,holdenk/spark,zero323/spark,jiangxb1987/spark,rikima/spark,zero323/spark,techaddict/spark,yanboliang/spark,highfei2011/spark,kiszk/spark,michalsenkyr/spark,hhbyyh/spark,dongjoon-hyun/spark,ueshin/apache-spark,lvdongr/spark,rezasafi/spark,skonto/spark,xuanyuanking/spark,HyukjinKwon/spark,zero323/spark,mahak/spark,skonto/spark,icexelloss/spark,xuanyuanking/spark,sahilTakiar/spark,chuckchen/spark,LantaoJin/spark,mahak/spark,shaneknapp/spark,ptkool/spark,rednaxelafx/apache-spark,LantaoJin/spark,ConeyLiu/spark,debugger87/spark,Aegeaner/spark,holdenk/spark,zero323/spark,hvanhovell/spark,zhouyejoe/spark,debugger87/spark,srowen/spark,bdrillard/spark,tengpeng/spark,nchammas/spark,hvanhovell/spark,ptkool/spark,mahak/spark,yanboliang/spark,apache/spark,JoshRosen/spark,icexelloss/spark,maropu/spark,LantaoJin/spark,taroplus/spark,goldmedal/spark,zzcclp/spark,eyalfa/spark,gengliangwang/spark,BryanCutler/spark,ahnqirage/spark,jkbradley/spark,rikima/spark,actuaryzhang/spark,techaddict/spark,aosagie/spark,WeichenXu123/spark,icexelloss/spark,bdrillard/spark,techaddict/spark,wzhfy/spark,cloud-fan/spark,wangmiao1981/spark,jkbradley/spark,kevinyu98/spark,debugger87/spark,shaneknapp/spark,JoshRosen/spark,holdenk/spark,bravo-zhang/spark,jiangxb1987/spark,bravo-zhang/spark,wangyum/spark,nchammas/spark,yanboliang/spark,BryanCutler/spark,HyukjinKwon/spark,shuangshuangwang/spark,guoxiaolongzte/spark,kevinyu98/spark,bravo-zhang/spark,kiszk/spark,WindCanDie/spark,ptkool/spark,zuotingbing/spark,darionyaphet/spark,shaneknapp/spark,bravo-zhang/spark,LantaoJin/spark,Aegeaner/spark,BryanCutler/spark,rikima/spark,wzhfy/spark,WeichenXu123/spark,WindCanDie/spark,vinodkc/spark,lxsmnv/spark,taroplus/spark,eyalfa/spark,lxsmnv/spark,skonto/spark,dbtsai/spark,guoxiaolongzte/spark,hhbyyh/spark,jiangxb1987/spark,debugger87/spark,guoxiaolongzte/spark,ahnqirage/spark,goldmedal/spark,srowen/spark,chuckchen/spark,rekhajoshm/spark,facaiy/spark,kevinyu98/spark,Aegeaner/spark,michalsenkyr/spark,wangmiao1981/spark,apache/spark,cloud-fan/spark,actuaryzhang/spark,guoxiaolongzte/spark,icexelloss/spark,rednaxelafx/apache-spark,caneGuy/spark,milliman/spark,gengliangwang/spark,aosagie/spark,jkbradley/spark,jiangxb1987/spark,michalsenkyr/spark,dongjoon-hyun/spark,cloud-fan/spark,ptkool/spark,apache/spark,srowen/spark,highfei2011/spark,zhouyejoe/spark,skonto/spark,kiszk/spark,spark-test/spark,highfei2011/spark,maropu/spark,darionyaphet/spark,ptkool/spark,HyukjinKwon/spark,kiszk/spark,sahilTakiar/spark,wzhfy/spark,guoxiaolongzte/spark,lvdongr/spark,ConeyLiu/spark,dbtsai/spark,actuaryzhang/spark,skonto/spark,wzhfy/spark,rekhajoshm/spark,shuangshuangwang/spark,apache/spark,darionyaphet/spark,zero323/spark,michalsenkyr/spark,wangyum/spark,rezasafi/spark,ahnqirage/spark,caneGuy/spark,spark-test/spark,jkbradley/spark,hvanhovell/spark,caneGuy/spark,facaiy/spark,facaiy/spark,wangmiao1981/spark,eyalfa/spark,cloud-fan/spark,zhouyejoe/spark,cloud-fan/spark,taroplus/spark,shaneknapp/spark,holdenk/spark,chuckchen/spark,rikima/spark,wangmiao1981/spark,shuangshuangwang/spark,tengpeng/spark,yanboliang/spark,shaneknapp/spark,Aegeaner/spark,mdespriee/spark,maropu/spark,witgo/spark,aosagie/spark,pgandhi999/spark,eyalfa/spark,gengliangwang/spark,dongjoon-hyun/spark,rezasafi/spark,gengliangwang/spark,sahilTakiar/spark,shuangshuangwang/spark,wzhfy/spark,maropu/spark,dbtsai/spark,yanboliang/spark,LantaoJin/spark,hvanhovell/spark,pgandhi999/spark,bdrillard/spark,zzcclp/spark,milliman/spark,zzcclp/spark,WindCanDie/spark,srowen/spark,ueshin/apache-spark,yanboliang/spark,holdenk/spark,ConeyLiu/spark,ueshin/apache-spark,wangyum/spark,jkbradley/spark,lvdongr/spark,zhouyejoe/spark,shuangshuangwang/spark,WeichenXu123/spark,mahak/spark,ahnqirage/spark,vinodkc/spark,rednaxelafx/apache-spark,JoshRosen/spark,wangmiao1981/spark,bdrillard/spark,pgandhi999/spark,spark-test/spark,ConeyLiu/spark,holdenk/spark,zuotingbing/spark,hvanhovell/spark,sahilTakiar/spark,mdespriee/spark,goldmedal/spark,HyukjinKwon/spark,wangyum/spark,wangyum/spark,guoxiaolongzte/spark,Aegeaner/spark,dbtsai/spark,nchammas/spark,icexelloss/spark,mdespriee/spark,dongjoon-hyun/spark,goldmedal/spark,techaddict/spark,vinodkc/spark,darionyaphet/spark,taroplus/spark,aosagie/spark,tejasapatil/spark,Aegeaner/spark,rezasafi/spark,spark-test/spark,debugger87/spark,zuotingbing/spark,facaiy/sp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