Update topnav.html

Author: talbring

_includes/topnav.html

@@ -23,7 +23,7 @@
             </ul>
             <div class="navbar-right">
                 <ul class="nav navbar-nav">
-                    <li><a href="https://join.slack.com/t/su2devteam/shared_invite/zt-af0uuqf8-8XNExKMV9G~UVsnkvi5uVA"><i class="fab fa-slack-hash" aria-hidden="true"></i></a></li>
+                    <li><a href="https://join.slack.com/t/su2devteam/shared_invite/zt-af0uuqf8-8XNExKMV9G~UVsnkvi5uVA"><i class="fa fa-slack" aria-hidden="true"></i></a></li>
                     <li><a href="{{ site.git_address }}"><i class="fa fa-github" aria-hidden="true"></i></a></li>
                     <li><a href="https://twitter.com/intent/user?screen_name=su2code"><i class="fa fa-twitter" aria-hidden="true"></i></a></li>
                     <li><a href="https://www.facebook.com/su2code/"><i class="fa fa-facebook" aria-hidden="true"></i></a></li>