Replace link to defunct Google Group with link to project Issues page

Author: rcaden

templates/error.tmpl

@@ -5,7 +5,7 @@
 <p>Possible causes:</p>
 
 <ol>
-<li>The address may be incorrect.  Make sure the address is spelled correctly.  Try loading the feed directly in your browser to make sure a feed exists at that address.</li>
-<li>The feed may be temporarily unavailable.  The server may be down, or too slow.  Try again later.</li>
-<li>The validator may be busted.  If the feed exists, the server is fine, and the problem is reproducible, let us know on the <a href="http://groups.google.com/group/feedvalidator-users">feedvalidator-users</a> mailing list.</li>
+<li>The address may be incorrect. Make sure the address is spelled correctly. Try loading the feed directly in your browser to make sure a feed exists at that address.</li>
+<li>The feed may be temporarily unavailable. The server may be down, or too slow. Try again later.</li>
+<li>The validator may be busted. If the feed exists, the server is fine, and the problem is reproducible, let us know on the <a href="https://github.com/rubys/feedvalidator/issues">Feed Validator Issues page/a> on GitHub.</li>
 </ol>