Merge pull request #1981 from sanshirookazaki/fix-slack-link

Fix slack link

Author: mat007

CONTRIBUTING.md

@@ -83,7 +83,7 @@ don't get discouraged! Our contributor's guide explains
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     <td>Community Slack</td>
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-      The Docker Community has a dedicated Slack chat to discuss features and issues.  You can sign-up <a href="https://community.docker.com/registrations/groups/4316" 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://dockercommunity.slack.com/ssb/redirect" target="_blank">with this link</a>.
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