[MINOR][DOCS] Fix the link to the pickle module page in RDD Programming Guide
[MINOR][DOCS] Fix the link to the pickle module page in RDD Programming Guide

### What changes were proposed in this pull request?
This pull request changes the link to the pickle module page from https://docs.python.org/2/library/pickle.html to https://docs.python.org/3/library/pickle.html in RDD Programming Guide.

### Why are the changes needed?
Since Python 2 is no longer supported and it is preferable to refer to the pickle module page of Python 3.

### Does this PR introduce _any_ user-facing change?
Yes.
Before: the `Pickle` link's destination page was https://docs.python.org/2/library/pickle.html
After: the `Pickle` link's destination page is https://docs.python.org/3/library/pickle.html

### How was this patch tested?
By building the documentation site and check the link's destination page is changed correctly in the local environment.

Closes #30081 from kjmrknsn/docs-fix-pickle-link.

Authored-by: Keiji Yoshida <08968e436dedf3f17c3abc3e5ec6f5e3c457c110@gmail.com>
Signed-off-by: HyukjinKwon <2680965ad5aae29275c8fb10150cb10f5554e78b@apache.org>

rednaxelafx/apache-spark,holdenk/spark,gengliangwang/spark,mahak/spark,witgo/spark,JoshRosen/spark,shuangshuangwang/spark,shuangshuangwang/spark,dongjoon-hyun/spark,srowen/spark,wangyum/spark,mahak/spark,witgo/spark,shuangshuangwang/spark,witgo/spark,srowen/spark,xuanyuanking/spark,ueshin/apache-spark,maropu/spark,mahak/spark,wangyum/spark,zero323/spark,maropu/spark,taroplus/spark,zero323/spark,jiangxb1987/spark,apache/spark,BryanCutler/spark,jiangxb1987/spark,taroplus/spark,wangmiao1981/spark,witgo/spark,vinodkc/spark,BryanCutler/spark,vinodkc/spark,zzcclp/spark,HyukjinKwon/spark,shuangshuangwang/spark,taroplus/spark,shaneknapp/spark,chuckchen/spark,BryanCutler/spark,HyukjinKwon/spark,shaneknapp/spark,mahak/spark,hvanhovell/spark,srowen/spark,rednaxelafx/apache-spark,wangmiao1981/spark,chuckchen/spark,vinodkc/spark,shaneknapp/spark,zero323/spark,apache/spark,cloud-fan/spark,HyukjinKwon/spark,milliman/spark,witgo/spark,zzcclp/spark,srowen/spark,gengliangwang/spark,vinodkc/spark,chuckchen/spark,xuanyuanking/spark,chuckchen/spark,nchammas/spark,wangmiao1981/spark,WeichenXu123/spark,xuanyuanking/spark,HyukjinKwon/spark,milliman/spark,witgo/spark,WeichenXu123/spark,cloud-fan/spark,jiangxb1987/spark,zzcclp/spark,wangyum/spark,JoshRosen/spark,ueshin/apache-spark,BryanCutler/spark,gengliangwang/spark,rednaxelafx/apache-spark,chuckchen/spark,dongjoon-hyun/spark,jiangxb1987/spark,rednaxelafx/apache-spark,maropu/spark,wangyum/spark,maropu/spark,maropu/spark,JoshRosen/spark,holdenk/spark,zzcclp/spark,jiangxb1987/spark,holdenk/spark,shaneknapp/spark,wangmiao1981/spark,zzcclp/spark,zzcclp/spark,apache/spark,cloud-fan/spark,shuangshuangwang/spark,gengliangwang/spark,maropu/spark,maropu/spark,mahak/spark,xuanyuanking/spark,dongjoon-hyun/spark,taroplus/spark,milliman/spark,JoshRosen/spark,gengliangwang/spark,rednaxelafx/apache-spark,apache/spark,dongjoon-hyun/spark,srowen/spark,hvanhovell/spark,srowen/spark,shaneknapp/spark,nchammas/spark,ueshin/apache-spark,hvanhovell/spark,wangyum/spark,zero323/spark,hvanhovell/spark,srowen/spark,nchammas/spark,WeichenXu123/spark,cloud-fan/spark,xuanyuanking/spark,hvanhovell/spark,BryanCutler/spark,rednaxelafx/apache-spark,cloud-fan/spark,wangmiao1981/spark,JoshRosen/spark,JoshRosen/spark,gengliangwang/spark,witgo/spark,shuangshuangwang/spark,nchammas/spark,WeichenXu123/spark,chuckchen/spark,mahak/spark,nchammas/spark,xuanyuanking/spark,wangmiao1981/spark,chuckchen/spark,mahak/spark,holdenk/spark,zzcclp/spark,HyukjinKwon/spark,nchammas/spark,taroplus/spark,holdenk/spark,nchammas/spark,taroplus/spark,hvanhovell/spark,taroplus/spark,milliman/spark,holdenk/spark,rednaxelafx/apache-spark,apache/spark,ueshin/apache-spark,cloud-fan/spark,vinodkc/spark,zero323/spark,JoshRosen/spark,BryanCutler/spark,wangyum/spark,xuanyuanking/spark,vinodkc/spark,ueshin/apache-spark,HyukjinKwon/spark,milliman/spark,jiangxb1987/spark,vinodkc/spark,zero323/spark,zero323/spark,holdenk/spark,dongjoon-hyun/spark,ueshin/apache-spark,dongjoon-hyun/spark,apache/spark,HyukjinKwon/spark,WeichenXu123/spark,shaneknapp/spark,shuangshuangwang/spark,milliman/spark,gengliangwang/spark,ueshin/apache-spark,wangyum/spark,dongjoon-hyun/spark,jiangxb1987/spark,apache/spark,BryanCutler/spark,WeichenXu123/spark,wangmiao1981/spark,cloud-fan/spark,hvanhovell/spark,WeichenXu123/spark,shaneknapp/spark,milliman/spark