Fixed the tooltip message for slack notifier

public/app/features/alerting/partials/notification_edit.html

@@ -71,7 +71,7 @@
           data-placement="right">
           data-placement="right">
         </input>
         </input>
         <info-popover mode="right-absolute">
         <info-popover mode="right-absolute">
-          Override default channel or user, use #channel-name or &lt;@username&gt;&lt;/@username&gt;
+          Override default channel or user, use #channel-name or @username
         </info-popover>
         </info-popover>
       </div>
       </div>
     </div>
     </div>