[SPARK-20849][DOC][FOLLOWUP] Document R DecisionTree - Link Classification Example
[SPARK-20849][DOC][FOLLOWUP] Document R DecisionTree - Link Classification Example

## What changes were proposed in this pull request?
in https://github.com/apache/spark/pull/18067, only the regression example is linked

this pr link decision tree classification example to the doc

ping felixcheung

## How was this patch tested?
local build of docs

![default](https://user-images.githubusercontent.com/7322292/33922857-9b00fdd0-e008-11e7-92c2-85a3de52ea8f.png)

Author: Zheng RuiFeng <ruifengz@foxmail.com>

Closes #19963 from zhengruifeng/r_examples.

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