Fix docs for scheduled event's cover

[honzajavorek]

Summary

When trying to figure out this problem https://stackoverflow.com/q/72134026/325365 (my own question from some time ago), I started reading code and it seems that the edit method actually takes bytes, not an asset! That would perhaps solve my problem. In this PR I'm trying to sync the docs with actual implementation.

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.