community: Update mattermost manual URL

Signed-off-by: Philippe Coval <rzr@users.sf.net>

community-chat-platform.rst

@@ -45,4 +45,4 @@ To join/explore other a public channel, do the following:
 #. Select **Join** to join a specific communication channel.
 
 For more details on using our chat platform powered by Mattermost, check out the
-`Mattermost User’s Guide <https://docs.mattermost.com/guides/user.html>`_.
+`Mattermost User’s Guide <https://docs.mattermost.com/guides/messaging.html>`_.