Change slack link in the other two places

...maybe we should make that easier?  Although this link
is not supposed to expire so it's probably fine as is.

Author: handrews

_includes/footer.html

@@ -12,7 +12,7 @@ <h2 class="footer-heading">{{ site.title | escape }}</h2>
             {% include icon-github.html username=site.github_username %}
           </li>
           {% endif %}
-          <li>Discussion: <a href="https://join.slack.com/t/json-schema/shared_invite/enQtMjk1NDcyNDI2NTAwLTcyYmYwMjdmMmUxNzZjYzIxNGU2YjdkNzdlOGZiNjIwNDI2M2Y3NmRkYjA4YmMwODMwYjgyOTFlNWZjZjAyNjg">Slack</a> | <a href="https://groups.google.com/forum/#!forum/json-schema">Google Groups</a></li>
+          <li>Discussion: <a href="https://join.slack.com/t/json-schema/shared_invite/enQtNjc5NTk0MzkzODg5LTVlZGIxNmVhMGY2MWFlYTdiNDQ5NWFiZGUwOThhNmYxZDE0YzA5YjRiOTA5MGY4ZTZlZGZhZDFmYTY4NWM2N2Y">Slack</a> | <a href="https://groups.google.com/forum/#!forum/json-schema">Google Groups</a></li>
           <li>Site edits: <a href="https://github.com/json-schema-org/json-schema-org.github.io">GitHub repo for site</a></li>
         </ul>
       </div>

index.md

@@ -97,4 +97,4 @@ Questions? Feeling helpful? Get involved on:
 
 * [GitHub](http://github.com/json-schema-org/json-schema-spec)
 * [Google Groups](https://groups.google.com/forum/#!forum/json-schema)
-* [Slack](https://join.slack.com/t/json-schema/shared_invite/enQtMjk1NDcyNDI2NTAwLTcyYmYwMjdmMmUxNzZjYzIxNGU2YjdkNzdlOGZiNjIwNDI2M2Y3NmRkYjA4YmMwODMwYjgyOTFlNWZjZjAyNjg)
+* [Slack](https://join.slack.com/t/json-schema/shared_invite/enQtNjc5NTk0MzkzODg5LTVlZGIxNmVhMGY2MWFlYTdiNDQ5NWFiZGUwOThhNmYxZDE0YzA5YjRiOTA5MGY4ZTZlZGZhZDFmYTY4NWM2N2Y)