Fix broken 'join slack' url in README.md

diff --git a/README.md b/README.md
index 6e12ccb..c7e7afe 100644
--- a/README.md
+++ b/README.md
@@ -1,7 +1,7 @@
 # openwhisk-slackinvite
 Simple webpage for invitations for Apache OpenWhisk Slack Community
 
-Join our Slack https://openwhisk.github.io/openwhisk-slackinvite
+Join our Slack https://openwhisk.apache.org/slack.html
 
 Backend is implemented as a serverless action with api gateway route