doc: mailing list is deprecated

Author: tshepang

src/doc/guide.md

@@ -101,7 +101,7 @@ you can access through
 [Mibbit](http://chat.mibbit.com/?server=irc.mozilla.org&channel=%23rust). Click
 that link, and you'll be chatting with other Rustaceans (a silly nickname we
 call ourselves), and we can help you out. Other great resources include [our
-mailing list](https://mail.mozilla.org/listinfo/rust-dev), [the /r/rust
+forum](http://discuss.rust-lang.org/), [the /r/rust
 subreddit](http://www.reddit.com/r/rust), and [Stack
 Overflow](http://stackoverflow.com/questions/tagged/rust).