chore(readme): update broken Slack link (patternfly#12547)

The slack.patternfly.org URL returns a 404. Replaced with patternfly.slack.com, matching the fix in patternfly-org#3364.

Fixes patternfly#12546.

Author: Axel-DaMage

README.md

@@ -5,7 +5,7 @@
 
 This project provides a set of React components for the [PatternFly project](https://patternfly.org).  
 
-**Community:** [PatternFly website](https://www.patternfly.org) | [Slack](https://slack.patternfly.org) | [Medium](https://medium.com/patternfly) | [Mailing list](https://www.redhat.com/mailman/listinfo/patternfly)
+**Community:** [PatternFly website](https://www.patternfly.org) | [Slack](https://patternfly.slack.com) | [Medium](https://medium.com/patternfly) | [Mailing list](https://www.redhat.com/mailman/listinfo/patternfly)
 
 ### Table of contents
 1. [PatternFly React packages](#patternfly-react-packages)