[SPARK-19797][DOC] ML pipeline document correction
[SPARK-19797][DOC] ML pipeline document correction

## What changes were proposed in this pull request?
Description about pipeline in this paragraph is incorrect https://spark.apache.org/docs/latest/ml-pipeline.html#how-it-works

> If the Pipeline had more **stages**, it would call the LogisticRegressionModel’s transform() method on the DataFrame before passing the DataFrame to the next stage.

Reason: Transformer could also be a stage. But only another Estimator will invoke an transform call and pass the data to next stage. The description in the document misleads ML pipeline users.

## How was this patch tested?
This is a tiny modification of **docs/ml-pipelines.md**. I jekyll build the modification and check the compiled document.

Author: Zhe Sun <ymwdalex@gmail.com>

Closes #17137 from ymwdalex/SPARK-19797-ML-pipeline-document-correction.

rekhajoshm/spark,dbtsai/spark,saturday-shi/spark,eyalfa/spark,1haodian/spark,liyichao/spark,debugger87/spark,shubhamchopra/spark,janewangfb/spark,zuotingbing/spark,adrian-ionescu/apache-spark,caneGuy/spark,akopich/spark,apache/spark,akopich/spark,taroplus/spark,esi-mineset/spark,skonto/spark,byakuinss/spark,esi-mineset/spark,ueshin/apache-spark,wangyum/spark,tengpeng/spark,narahari92/spark,guoxiaolongzte/spark,darionyaphet/spark,taroplus/spark,setjet/spark,aosagie/spark,jiangxb1987/spark,mike0sv/spark,skonto/spark,saturday-shi/spark,jiangxb1987/spark,janewangfb/spark,poffuomo/spark,nilsgrabbert/spark,sahilTakiar/spark,publicRoman/spark,SHASHANKB/spark,mike0sv/spark,aokolnychyi/spark,kissmetrics/spark,cloud-fan/spark,rikima/spark,mzl9039/spark,andrewor14/spark,zuotingbing/spark,alunarbeach/spark,janewangfb/spark,map222/spark,jkbradley/spark,bdrillard/spark,bOOm-X/spark,debugger87/spark,ueshin/apache-spark,zero323/spark,dbtsai/spark,poffuomo/spark,tejasapatil/spark,narahari92/spark,actuaryzhang/spark,taroplus/spark,hvanhovell/spark,sureshthalamati/spark,zzcclp/spark,markhamstra/spark,Aegeaner/spark,shubhamchopra/spark,liutang123/spark,guoxiaolongzte/spark,MLnick/spark,caneGuy/spark,caneGuy/spark,stanzhai/spark,bOOm-X/spark,witgo/spark,shaneknapp/spark,hhbyyh/spark,icexelloss/spark,brad-kaiser/spark,cin/spark,aray/spark,LantaoJin/spark,MLnick/spark,techaddict/spark,spark-test/spark,icexelloss/spark,ioana-delaney/spark,liyichao/spark,adrian-ionescu/apache-spark,pgandhi999/spark,lvdongr/spark,ajaysaini725/spark,SHASHANKB/spark,WeichenXu123/spark,debugger87/spark,zuotingbing/spark,bdrillard/spark,eyalfa/spark,nilsgrabbert/spark,vinodkc/spark,dongjoon-hyun/spark,stanzhai/spark,akopich/spark,sachintyagi22/spark,map222/spark,nilsgrabbert/spark,ahnqirage/spark,cin/spark,tengpeng/spark,actuaryzhang/spark,liyichao/spark,spark-test/spark,jlopezmalla/spark,jlopezmalla/spark,joseph-torres/spark,maropu/spark,ddna1021/spark,BryanCutler/spark,esi-mineset/spark,rezasafi/spark,joseph-torres/spark,shuangshuangwang/spark,apache-spark-on-k8s/spark,sahilTakiar/spark,xflin/spark,aray/spark,tejasapatil/spark,ptkool/spark,yanboliang/spark,wzhfy/spark,zhouyejoe/spark,bOOm-X/spark,LantaoJin/spark,someorz/spark,JoshRosen/spark,dhruve/spark,hhbyyh/spark,ueshin/apache-spark,spark-test/spark,nlalevee/spark,kiszk/spark,jlopezmalla/spark,sureshthalamati/spark,mahak/spark,jrshust/spark,rekhajoshm/spark,cin/spark,someorz/spark,HyukjinKwon/spark,LantaoJin/spark,minixalpha/spark,yanboliang/spark,panfengfeng/spark,mahak/spark,janewangfb/spark,jlopezmalla/spark,cin/spark,minixalpha/spark,apache/spark,rikima/spark,bravo-zhang/spark,wangmiao1981/spark,goldmedal/spark,ericvandenbergfb/spark,ConeyLiu/spark,adrian-ionescu/apache-spark,huang1900/spark,1haodian/spark,chuckchen/spark,rednaxelafx/apache-spark,LantaoJin/spark,someorz/spark,someorz/spark,apache-spark-on-k8s/spark,techaddict/spark,liutang123/spark,aokolnychyi/spark,caneGuy/spark,mahak/spark,jianran/spark,saltstar/spark,aosagie/spark,ron8hu/spark,BryanCutler/spark,icexelloss/spark,jiangxb1987/spark,rikima/spark,ajaysaini725/spark,mdespriee/spark,matthewfranglen/spark,JoshRosen/spark,ron8hu/spark,taroplus/spark,alunarbeach/spark,andrewor14/spark,patrick-nicholson/spark,wzhfy/spark,skonto/spark,lvdongr/spark,kiszk/spark,kissmetrics/spark,rednaxelafx/apache-spark,ConeyLiu/spark,kiszk/spark,kevinyu98/spark,debugger87/spark,skonto/spark,eyalfa/spark,ioana-delaney/spark,tengpeng/spark,nlalevee/spark,ptkool/spark,minixalpha/spark,jlopezmalla/spark,holdenk/spark,huang1900/spark,eyalfa/spark,lxsmnv/spark,hhbyyh/spark,MLnick/spark,markhamstra/spark,huang1900/spark,ptkool/spark,maropu/spark,guoxiaolongzte/spark,holdenk/spark,rednaxelafx/apache-spark,dongjoon-hyun/spark,chuckchen/spark,joseph-torres/spark,nchammas/spark,stanzhai/spark,saltstar/spark,nilsgrabbert/spark,bdrillard/spark,srowen/spark,xuanyuanking/spark,dhruve/spark,rezasafi/spark,1haodian/spark,michalsenkyr/spark,ajaysaini725/spark,stanzhai/spark,HyukjinKwon/spark,apache-spark-on-k8s/spark,JerryLead/spark,nchammas/spark,WindCanDie/spark,1haodian/spark,skonto/spark,aray/spark,map222/spark,saltstar/spark,wangyum/spark,jrshust/spark,mzl9039/spark,someorz/spark,zzcclp/spark,icexelloss/spark,alunarbeach/spark,poffuomo/spark,rezasafi/spark,JerryLead/spark,adrian-ionescu/apache-spark,WeichenXu123/spark,actuaryzhang/spark,sachintyagi22/spark,eyalfa/spark,jiangxb1987/spark,wangyum/spark,apache/spark,akopich/spark,bravo-zhang/spark,zero323/spark,aosagie/spark,lvdongr/spark,mike0sv/spark,lvdongr/spark,dhruve/spark,pgandhi999/spark,sahilTakiar/spark,akopich/spark,shaneknapp/spark,kevinyu98/spark,kissmetrics/spark,huang1900/spark,HyukjinKwon/spark,nlalevee/spark,mdespriee/spark,bravo-zhang/spark,panfengfeng/spark,rezasafi/spark,1haodian/spark,setjet/spark,lxsmnv/spark,dongjoon-hyun/spark,zhouyejoe/spark,taroplus/spark,shubhamchopra/spark,dhruve/spark,sachintyagi22/spark,WindCanDie/spark,jrshust/spark,dbtsai/spark,liyichao/spark,kevinyu98/spark,wangmiao198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