Change slack invite link.

This one should never expire!

Author: Relequestual

_includes/header.html

@@ -25,7 +25,7 @@
             <a class="page-link" href="{{ my_page.url | relative_url }}">{{ my_page.title | escape }}</a>
             {%- endif -%}
           {% endfor -%}
-          <a class="page-link" href="https://join.slack.com/t/json-schema/shared_invite/enQtMjk1NDcyNDI2NTAwLTcyYmYwMjdmMmUxNzZjYzIxNGU2YjdkNzdlOGZiNjIwNDI2M2Y3NmRkYjA4YmMwODMwYjgyOTFlNWZjZjAyNjg">
+          <a class="page-link" href="https://join.slack.com/t/json-schema/shared_invite/enQtNjc5NTk0MzkzODg5LTVlZGIxNmVhMGY2MWFlYTdiNDQ5NWFiZGUwOThhNmYxZDE0YzA5YjRiOTA5MGY4ZTZlZGZhZDFmYTY4NWM2N2Y">
             Discussion
           </a>
         </div>