Fix mojo mailing list

Fixes the address of the chromium-mojo mailing list. Also makes it link
to the Google Groups page for consistency with other Mojo docs.

Bug: None
Change-Id: Ic2230d996efab8b75c41c1a0d73f889d5c1124bb
Reviewed-on: https://chromium-review.googlesource.com/c/1462147
Auto-Submit: Michael Giuffrida <michaelpg@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#630701}

docs/security/mojo.md

@@ -5,7 +5,8 @@ abstractions can make it easier to write code that makes interprocess calls,
 but can also add significant complexity. Below are some recommendation from
 Mojo and IPC reviewers for best practices.
 
-For questions, concerns, or suggestions, reach out to <mojo@chromium.org>.
+For questions, concerns, or suggestions, reach out to
+[chromium-mojo@chromium.org](https://groups.google.com/a/chromium.org/forum/#!forum/chromium-mojo).
 
 > For legacy IPC, please see [security tips for IPC][security-tips-for-ipc].