Add channel_id to RawMessageUpdateEvent

[Heath123]

Summary

Before this a RawMessageUpdateEvent had no channel_id property. It could, however, be accessed with .data["channel_id"]

This is my first PR to anything ever so it's just a tiny change. Sorry if something is wrong.

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes. (I think it updates automatically for what I did)
• 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, ...)

[Heath123]

Sometimes it isn't provided apparently

[Heath123]

Oh, it is always provided. Sorry