Removed the slack archives

Removed the slack archive linke because it doesn't exist anymore.

Signed-off-by: kaffarell <[email protected]>

CONTRIBUTING.md

@@ -101,9 +101,8 @@ the contributors guide.
     <td>
       <p>
         Register for the Docker Community Slack at
-	<a href="https://community.docker.com/registrations/groups/4316" target="_blank">https://community.docker.com/registrations/groups/4316</a>.
+	<a href="https://dockr.ly/slack" target="_blank">https://dockr.ly/slack</a>.
         We use the #moby-project channel for general discussion, and there are separate channels for other Moby projects such as #containerd.
-	Archives are available at <a href="https://dockercommunity.slackarchive.io/" target="_blank">https://dockercommunity.slackarchive.io/</a>.
       </p>
     </td>
   </tr>