DOC: Fixed examples in `pandas/core/aggregation.py`
- [ ] closes #xxxx
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

> Probably better to split the changes to the examples from the changes to the typing. I guess they're unrelated.

@datapythonista by "split" you mean, separate PRs? Yes, I mean separate PRs > Probably better to split the changes to the examples from the changes to the typing. I guess they're unrelated.

IMO they are related. see https://github.com/pandas-dev/pandas/pull/33263#discussion_r403133623 Thanks @MomIsBestFriend 