message: docs: update dev chat link
  The gitter room is not used anymore.
  This commit updates CONTRIBUTING.md to guide the reader to the
  discord channel.
diff: @@ -12,14 +12,14 @@ expert, we can use your help.
  This guide will help you get started. **Do not let this guide intimidate you**.
  It should be considered a map to help you navigate the process.
  -You may also get help with contributing in the [dev channel][dev], please join
  -us!
  +You may also get help with contributing in [Tokio's Discord server][discord],
  +`#tracing-dev` channel. Please join us!
  Tracing is a part of the [Tokio][tokio] project, and follows the project's
  guidelines for contributing. This document is based on the
  [`CONTRIBUTING.md` file][tokio-contrib] in the `tokio-rs/tokio` repository.
  -[dev]: https://gitter.im/tokio-rs/dev
  +[discord]: https://discord.gg/tokio
  [tokio]: https://tokio.rs
  [tokio-contrib]: https://github.com/tokio-rs/tokio/blob/master/CONTRIBUTING.md
  