Update Slack link

Author: nlharris

_includes/header.html

@@ -137,7 +137,7 @@
                                 Mail List</a>
                         </li>
                         <li>
-                            <a class="dropdown-item" href="https://join.slack.com/t/obo-communitygroup/shared_invite/zt-1oq48ttk7-kKo0i6TwntYtAq~Jcjjg4g">OBO
+                            <a class="dropdown-item" href="https://join.slack.com/t/obo-communitygroup/shared_invite/zt-3gs80gmrk-9yZT913GViLUg04pgE3wOA">OBO
                                 Community Slack channel</a>
                         </li>
                         <li>