Changed the link to Boost-Users after the switch to Mailman

[SVN r20035]

more/mailing_lists.htm

@@ -141,7 +141,7 @@
     answers in a forum dedicated to your problem area.  This list is
     relatively low volume (less than 100 per month).  Subscribe or
     unsubscribe at the <a href=
-    "http://groups.yahoo.com/group/Boost-Users">Boost Users list home
+    "http://lists.boost.org/mailman/listinfo.cgi/boost-users">Boost Users list home
     page</a>.</p>
 
     <p>For those who prefer to participate via an NNTP (<a name=