Update Slack public URL (#413)

Author: rosemarymarano

documentation/1.11/themes/hugo-theme-learn/layouts/partials/menu-footer.html

@@ -1,4 +1,4 @@
 <p>Created with <i class="fas fa-heart"></i> from <a href="https://www.oracle.com">Oracle</a></p>
 <p>&nbsp</p>
 <p><a href="https://github.com/oracle/weblogic-image-tool"><i class="fab fa-github"></i> GitHub repo</a></p>
-<p><a href="https://weblogic-slack-inviter.herokuapp.com/"><i class="fab fa-slack"></i> Public Slack #weblogic</a></p>
+<p><a href="https://join.slack.com/t/oracle-weblogic/shared_invite/zt-1lnz4kpci-WdY2gWfeJc5jS_a_1Z06MA"><i class="fab fa-slack"></i> Public Slack #weblogic</a></p>