Fix typo that causes several NaT methods to have incorrect docstrings
This block of code exists to make NaT raise ValueError for each of a bunch of datetime methods. This is supposed to attach the original docstring to the new function f. The typo _method_name instead of func_name means that a bunch of the docstrings are currently wrong.

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