Update slack invite link (#197)

Author: taknira

README.md

@@ -26,4 +26,4 @@ state of a transparent log.
 
 ## Support
 * Mailing list: https://groups.google.com/forum/#!forum/trillian-transparency
-- Slack: https://transparency-dev.slack.com/ ([invitation](https://join.slack.com/t/transparency-dev/shared_invite/zt-27pkqo21d-okUFhur7YZ0rFoJVIOPznQ))
+- Slack: https://transparency-dev.slack.com/ ([invitation](https://transparency.dev/slack/))