Improve formatting on slack / google groups links

There wasn't a space after the colon and the separation between
the two links wasn't very clear

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/enQtMjk1NDcyNDI2NTAwLTcyYmYwMjdmMmUxNzZjYzIxNGU2YjdkNzdlOGZiNjIwNDI2M2Y3NmRkYjA4YmMwODMwYjgyOTFlNWZjZjAyNjg">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>