message: Document `rspotify::sync` in changelog
diff: **Breaking changes**:
  - ([#325](https://github.com/ramsayleung/rspotify/pull/325)) The `auth_code`, `auth_code_pkce`, `client_creds`, `clients::base` and `clients::oauth` modules have been removed from the public API; you should access the same types from their parent modules instead
  +- ([#326](https://github.com/ramsayleung/rspotify/pull/326)) The `rspotify::clients::mutex` module has been renamed to `rspotify::sync`
  ## 0.11.5 (2022.03.28)
  