Contribute to issue #15580
- [ ] closes #15580
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

I'm new at contributing on Pandas and any project at all, and saw this generic "fix docs" issue. I made some very small changes that i think it helps users to understand better the `groupBy` usage. Please, feel free to judge my changes and request modifications. Thanks!