Fix Slack invite link in README (uber#2619)

* Update Slack invitation link

README.md

@@ -32,7 +32,7 @@ Try out [Cadence Web UI](https://github.com/uber/cadence-web) to view your workf
 
 We'd love your help in making Cadence great. Please review our [contribution guide](CONTRIBUTING.md).
 
-If you'd like to propose a new feature, first join the Cadence [discussion group](https://groups.google.com/d/forum/cadence-discussion) and [Slack channel](https://join.slack.com/t/uber-cadence/shared_invite/enQtNDczNTgxMjYxNDEzLTI5Yzc5ODYwMjg1ZmI3NmRmMTU1MjQ0YzQyZDc5NzMwMmM0NjkzNDE5MmM0NzU5YTlhMmI4NzIzMDhiNzFjMDM) to start a discussion and check if there are existing design discussions. Also peruse our [design docs](docs/design/index.md) in case a feature has been designed but not yet implemented. Once you're sure the proposal is not covered elsewhere, please follow our [proposal instructions](PROPOSALS.md).
+If you'd like to propose a new feature, first join the Cadence [discussion group](https://groups.google.com/d/forum/cadence-discussion) and [Slack channel](https://join.slack.com/t/uber-cadence/shared_invite/enQtNDczNTgxMjYxNDEzLTQyYjcxZDM2YTIxMTZkMzQ0NjgxYmI3OWY5ODhiOTliM2I5MzA4NTM4MjU4YzgzZDkwNGEzOTUzNTBlNDk3Yjc) to start a discussion and check if there are existing design discussions. Also peruse our [design docs](docs/design/index.md) in case a feature has been designed but not yet implemented. Once you're sure the proposal is not covered elsewhere, please follow our [proposal instructions](PROPOSALS.md).
 
 ## License