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

@datapythonista @jorisvandenbossche if any comments > @MomIsBestFriend can you add those files to the "DOCTESTS" section in code_checks.sh? (or are not yet all docstrings of the files passing?)

@jorisvandenbossche Yes, all the doctests are now passing, I have added it in 2a330b4 Thanks!