Update Slack link

(the Slack link on this home page was the generic one that only works if you're already a member - was there a reason we decided not to include the invite link here? if so, please remind me!)

Author: nlharris

index.html

@@ -30,7 +30,7 @@ <h4>Participate</h4>
             <li><a href="docs/COC.html">Code of Conduct</a></li>
             <li>
                 Join the <a href="https://groups.google.com/forum/#!forum/obo-discuss">OBO mailing list</a>
-                and the <a href="https://obo-communitygroup.slack.com/">OBO Community Slack workspace</a>
+                and the <a href="https://join.slack.com/t/obo-communitygroup/shared_invite/zt-3gs80gmrk-9yZT913GViLUg04pgE3wOA">OBO Community Slack workspace</a>
             </li>
             <li>
                 <a href="docs/OperationsCommittee.html">