CONTRIBUTING: replace IRC reference

Author: kaznovac

CONTRIBUTING.md

@@ -2,7 +2,7 @@
 
 Anybody who subscribes to the Google Group, is part of the PHP-FIG. As soon as
 you subscribe to the [mailing list](http://groups.google.com/group/php-fig/)
-and/or join the [IRC channel](https://www.php-fig.org/irc/) you are a PHP-FIG
+and/or join the [Discord Server](https://discord.gg/php-fig) you are a PHP-FIG
 Community Member, who can influence standards, make suggestions, give feedback,
 etc. Only PHP-FIG Voting Members can start or participate in votes, but the
 discussion and formation stages involve everyone.