DOC: Update pandas.core.groupby.GroupBy.pipe docstring
- [x] closes https://github.com/pandanistas/pandanistas_sprint_ui2020/issues/2
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
