readme: fix slack invite link (envoyproxy#19864)

Signed-off-by: Matt Klein <[email protected]>

Author: mattklein123

README.md

@@ -47,7 +47,7 @@ to find out more about the origin story and design philosophy of Envoy
 * [envoy-maintainers](https://groups.google.com/forum/#!forum/envoy-maintainers): Use this list
   to reach all core Envoy maintainers.
 * [Twitter](https://twitter.com/EnvoyProxy/): Follow along on Twitter!
-* [Slack](https://envoyproxy.slack.com/): Slack, to get invited go [here](https://envoyslack.cncf.io).
+* [Slack](https://envoyproxy.slack.com/): Slack, to get invited go [here](https://envoyproxy.io/slack).
   We have the IRC/XMPP gateways enabled if you prefer either of those. Once an account is created,
   connection instructions for IRC/XMPP can be found [here](https://envoyproxy.slack.com/account/gateways).
   * NOTE: Response to user questions is best effort on Slack. For a "guaranteed" response please email