Tiny improvement relating to an example for slash commands

[UP929312]

Summary

Currently the slash command example is really wordy, and doesn't mention in inability to respond twice,
this PR aims to help clear up how ctx.defer is used, and also mentions the aforementioned limitation of responding twice.

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.