Update Fix help message url for "None and Optional handling" section

[guywilsonjr]

Fixes #19251

Correct url in help message"None and Optional handling" group
Checklist:

• Read the Contributing Guidelines
• Add tests for all changed behaviour.
• If you can't add a test, please explain why and how you verified your changes work.

$ mypy -h
....
None and Optional handling:
  Adjust how values of type 'None' are handled. For more context on how mypy handles values of type 'None', see:
  https://mypy.readthedocs.io/en/stable/kinds_of_types.html#optional-types-and-the-none-type
...

• - Make sure CI passes.
• - Please do not force push to the PR once it has been reviewed.

[A5rocks]

Maybe it makes more sense to update the docs to support this old anchor, for people on older mypy versions?

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅