Meta: Refer to Matrix rather than Freenode in "About" (tc39#2533)

Author: bakkot

spec.html

@@ -84,7 +84,7 @@ <h1>Contributing to this Specification</h1>
       Community:
       <ul>
         <li>Discourse: <a href="https://es.discourse.group">https://es.discourse.group</a></li>
-        <li>IRC: <a href="ircs://irc.freenode.net:6667">#tc39</a> on <a href="https://freenode.net/kb/answer/chat">freenode</a></li>
+        <li>Chat: <a href="https://github.com/tc39/how-we-work/blob/HEAD/matrix-guide.md">Matrix</a></li>
         <li>Mailing <emu-not-ref>List</emu-not-ref> Archives: <a href="https://esdiscuss.org">https://esdiscuss.org/</a></li>
       </ul>
     </li>