Fix typo in voice server update state handling

[dolfies]

Summary

Fixes a typo in the voice server update state handling, where previous_token is checked twice instead of previous_endpoint.

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the 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, ...)