[fix] Change link to Slack workspace auto-invite (#18933)

Author: merlimat

README.md

@@ -240,7 +240,7 @@ Read https://pulsar.apache.org/contribute/setup-ide for setting up IntelliJ IDEA
 
 Pulsar slack channel at https://apache-pulsar.slack.com/
 
-You can self-register at https://apache-pulsar.herokuapp.com/
+You can self-register at https://communityinviter.com/apps/apache-pulsar/apache-pulsar
 
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