message: Fix link to Zulip chat room
diff: # Zulip
  -[Rust's Zulip](rust-lang.zulipchat.com) is used by a number of teams, notably
  +[Rust's Zulip](https://rust-lang.zulipchat.com) is used by a number of teams, notably
  the compiler, language, and library teams, along with their working groups.
  Zulip can be an unintuitive platform to get started with. To get started, take a
  