DOC: fixed typos in GroupBy document
DOC: fixed typos in GroupBy document

Author: Mortada Mehyar <mortada.mehyar@gmail.com>

Closes #13270 from mortada/docs and squashes the following commits:

a3dea39 [Mortada Mehyar] DOC: fixed typos in GroupBy document

zfrenchee/pandas,gfyoung/pandas,DGrady/pandas,zfrenchee/pandas,nmartensen/pandas,cbertinato/pandas,linebp/pandas,louispotok/pandas,harisbal/pandas,harisbal/pandas,jreback/pandas,winklerand/pandas,pandas-dev/pandas,DGrady/pandas,jreback/pandas,MJuddBooth/pandas,datapythonista/pandas,cbertinato/pandas,jmmease/pandas,zfrenchee/pandas,zfrenchee/pandas,MJuddBooth/pandas,toobaz/pandas,amolkahat/pandas,GuessWhoSamFoo/pandas,jreback/pandas,amolkahat/pandas,dsm054/pandas,gfyoung/pandas,toobaz/pandas,pratapvardhan/pandas,Winand/pandas,datapythonista/pandas,jmmease/pandas,GuessWhoSamFoo/pandas,winklerand/pandas,DGrady/pandas,Winand/pandas,cython-testbed/pandas,nmartensen/pandas,kdebrab/pandas,harisbal/pandas,gfyoung/pandas,jorisvandenbossche/pandas,gfyoung/pandas,DGrady/pandas,dsm054/pandas,toobaz/pandas,MJuddBooth/pandas,jorisvandenbossche/pandas,jreback/pandas,harisbal/pandas,linebp/pandas,GuessWhoSamFoo/pandas,cython-testbed/pandas,linebp/pandas,TomAugspurger/pandas,TomAugspurger/pandas,cython-testbed/pandas,louispotok/pandas,dsm054/pandas,DGrady/pandas,toobaz/pandas,TomAugspurger/pandas,winklerand/pandas,datapythonista/pandas,kdebrab/pandas,rs2/pandas,winklerand/pandas,jmmease/pandas,pratapvardhan/pandas,dsm054/pandas,gfyoung/pandas,Winand/pandas,nmartensen/pandas,pratapvardhan/pandas,cbertinato/pandas,MJuddBooth/pandas,amolkahat/pandas,datapythonista/pandas,nmartensen/pandas,GuessWhoSamFoo/pandas,kdebrab/pandas,kdebrab/pandas,pandas-dev/pandas,amolkahat/pandas,rs2/pandas,MJuddBooth/pandas,jmmease/pandas,rs2/pandas,louispotok/pandas,cbertinato/pandas,cython-testbed/pandas,jmmease/pandas,louispotok/pandas,zfrenchee/pandas,GuessWhoSamFoo/pandas,nmartensen/pandas,rs2/pandas,jmmease/pandas,toobaz/pandas,cython-testbed/pandas,pandas-dev/pandas,winklerand/pandas,linebp/pandas,winklerand/pandas,nmartensen/pandas,jreback/pandas,harisbal/pandas,pandas-dev/pandas,DGrady/pandas,kdebrab/pandas,dsm054/pandas,jorisvandenbossche/pandas,amolkahat/pandas,linebp/pandas,louispotok/pandas,linebp/pandas,pratapvardhan/pandas,Winand/pandas,cbertinato/pandas,Winand/pandas,jorisvandenbossche/pandas,TomAugspurger/pandas,Winand/pandas,pratapvardhan/pandas