Merge pull request #4972 from vvoland/community-slack

CONTRIBUTING.md: update Slack link

Author: neersighted

CONTRIBUTING.md

@@ -84,7 +84,7 @@ use for simple changes](https://docs.docker.com/opensource/workflow/make-a-contr
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     <td>Community Slack</td>
     <td>
-      The Docker Community has a dedicated Slack chat to discuss features and issues.  You can sign-up <a href="https://dockr.ly/slack" target="_blank">with this link</a>.
+      The Docker Community has a dedicated Slack chat to discuss features and issues.  You can sign-up <a href="https://dockr.ly/comm-slack" target="_blank">with this link</a>.
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   </tr>
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