fixed irc link

CONTRIBUTING.md

@@ -1,6 +1,7 @@
 # [Nextcloud](https://nextcloud.com) Android app
+
 [![irc](https://img.shields.io/badge/IRC-%23nextcloud%20on%20freenode-orange.svg)](https://webchat.freenode.net/?channels=nextcloud)
-[![irc](https://img.shields.io/badge/IRC-%23nextcloud-mobile%20on%20freenode-blue.svg)](https://webchat.freenode.net/?channels=nextcloud-mobile)
+[![irc](https://img.shields.io/badge/IRC-%23nextcloud--mobile%20on%20freenode-blue.svg)](https://webchat.freenode.net/?channels=nextcloud-mobile)
 
 
 # Index