Add missing fstrings in Local Calendar

[tkdrob]

Breaking change

Proposed change

I noticed this when trying to figure out why I could not update a mid-sequence event to a different day. The integration can't seem to handle this even though the UI says it is possible. Deleting and recreating a new event may be a good idea. This PR just makes sure the user has some actual information to go by as to why it does not work.

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• Dependency upgrade
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• New integration (thank you!)
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• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Additional information

• This PR fixes or closes issue: fixes #
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[home-assistant]

Hey there @allenporter, mind taking a look at this pull request as it has been labeled with an integration (local_calendar) you are listed as a code owner for? Thanks!

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[allenporter]

Good catch, thank you.

Deleting and recreating a new event may be a good idea

No, we need to preserve existing identifiers, or we may be partially editing without all event fields being passed on the request. Let's fix the underlying issue.