Updatign slack invite link

Author: pavanky

README.md

@@ -78,7 +78,7 @@ python -m arrayfire.tests
 
 ## Communication
 
-* [Slack Chat](https://join.slack.com/t/arrayfire-org/shared_invite/MjI1ODQ4NjI1MTM4LTE1MDI1NTgzOTctNzdjZmYyYWIwNA)
+* [Slack Chat](https://join.slack.com/t/arrayfire-org/shared_invite/MjI4MjIzMDMzMTczLTE1MDI5ODg4NzYtN2QwNGE3ODA5OQ)
 * [Google Groups](https://groups.google.com/forum/#!forum/arrayfire-users)
 
 ## Acknowledgements