message: Add missing link to release 0.18.1
diff: @@ -656,6 +656,7 @@ is based on [Keep a Changelog](https://keepachangelog.com).
  - Configuring colored terminal output should now print colored output.
  [Unreleased]: https://github.com/actor-framework/actor-framework/compare/0.18.0...master
  +[0.18.1]: https://github.com/actor-framework/actor-framework/releases/0.18.1
  [0.18.0]: https://github.com/actor-framework/actor-framework/releases/0.18.0
  [0.18.0-rc.1]: https://github.com/actor-framework/actor-framework/releases/0.18.0-rc.1
  [0.17.7]: https://github.com/actor-framework/actor-framework/compare/0.17.6...release/0.17
  