docs: fix SS06, PR08, RT02, RT03, SA04 in pandas.DatetimeIndex.indexe…
…r_at_time

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

 Fix : https://github.com/pandanistas/pandanistas_sprint_ui2020/issues/8
