Fix Typo & Type Error in LavalinkException

[Jiggly-Balls]

Description

This PR fixes a typo in the docs of wavelink.exception.LavalinkException class, where it mentions the attribute reason to hold the reason of the exception but it's the error attribute that holds the reason.

I've decided to only change to documentation name to match the attribute name instead of matching the attribute name to the mentioned name in the doc, to prevent any backward incompatibility.

The only thing I'm not fully sure of is the additional type annotation "fix" I made, the docs mention that the reason (error attr) can be None but the ErrorResponse typed dict & the error attribute in the LavalinkException class don't annotate it as None | str. So either I'm correct about the fix or the documentation is wrong about reason (error) being None | str.

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
  • I have updated the changelog with a quick recap of my changes.
• 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, ...)
• I have read and agree to the Developer Certificate of Origin for this contribution

P.S. There doesn't seem to be a changelog file in this repo... unless I'm terribly blind.