Make guild.audit_logs(limit) parameter optional

[r-hensley]

As written in the docs below the command:

limit: Optional[`int`]
            The number of entries to retrieve. If ``None`` retrieve all entries.

So the parameter should be Optional. Currently my IME returns the error Expected type 'int', got 'None' instead

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Checklist

• If code changes were made then they have been tested.
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• This PR fixes an issue.
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