[SPARK-21976][DOC] Fix wrong documentation for Mean Absolute Error.
[SPARK-21976][DOC] Fix wrong documentation for Mean Absolute Error.

## What changes were proposed in this pull request?

Fixed wrong documentation for Mean Absolute Error.

Even though the code is correct for the MAE:

```scala
Since("1.2.0")
  def meanAbsoluteError: Double = {
    summary.normL1(1) / summary.count
  }
```
In the documentation the division by N is missing.

## How was this patch tested?

All of spark tests were run.

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

Author: FavioVazquez <favio.vazquezp@gmail.com>
Author: faviovazquez <favio.vazquezp@gmail.com>
Author: Favio André Vázquez <favio.vazquezp@gmail.com>

Closes #19190 from FavioVazquez/mae-fix.

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