fix: update the slack channel url

Author: angegar

.github/ISSUE_TEMPLATE/-support-request.md

@@ -14,6 +14,6 @@ GitHub is not the right place for support requests.
 
 If you're looking for help, check our [docs](https://github.com/kubernetes-sigs/external-dns/tree/HEAD/docs).
 
-You can also post your question on the [Kubernetes Slack #external-dns](https://slack.k8s.io).
+You can also post your question on the [Kubernetes Slack #external-dns](https://slack.k8s.io/?channel=external-dns).
 
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