Documentation fix for method last_valid_index
Documentation was showing "Return index for first non-NA/null value" for both first_valid_index and last_valid_index methods. Fixed by switching "first" to "last"

- [ X ] closes #18564
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
