docs: mark Client.get_or_fetch_user() as a coro

[Sengolda]

Summary

Client.get_or_fetch_user() was not marked as a coro meaning it did not show the This function is a coroutine in the docs.

Information

• 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, ...).

Checklist

• I have searched the open pull requests for duplicates.
• 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.
• I have updated the changelog to include these changes.

[Lulalaby]

Thank u!

[pullapprove4]

Please add a changelog entry