DOC: Update groupby.rst
The previous description still had a conflict between the column name and index level

- [x] closes #16870 
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Thanks @Nikhil1O1 