Update slack invite link (kubeflow#1550)

The current kubeflow slack link is outdate. This change will match the slack invite link to the community guide on the website

https://www.kubeflow.org/docs/about/community/#slack-community-and-channels

CONTRIBUTING.md

@@ -44,6 +44,6 @@ The following should be viewed as Best Practices unless you know better ones (pl
 
 ## Get involved
 
-* [Slack](https://kubeflow.slack.com/join/shared_invite/zt-cpr020z4-PfcAue_2nw67~iIDy7maAQ)
+* [Slack](https://kubeflow.slack.com/join/shared_invite/zt-n73pfj05-l206djXlXk5qdQKs4o1Zkg#/)
 * [Twitter](http://twitter.com/kubeflow)
 * [Mailing List](https://groups.google.com/forum/#!forum/kubeflow-discuss)