doc fix for testing.assert_series_equal check_freq arg
This fixes `check_freq` arg docs for `assert_series_equal` like #38471 did for `assert_frame_equal`.

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