Update guild.py fetch_members() type hints

[IotaSpencer]

Actually type hint for None OR int using Union[int, None]

Summary

Fix type hinting for fetch_members so IDEs don't complain that it expects just an int

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• If type: ignore comments were used, a comment is also left explaining why
• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, typehinting, examples, ...)