Rename variable (matrix_user_groupname -> matrix_group_name)

docs/configuring-playbook-sygnal.md

@@ -50,7 +50,7 @@ aux_file_definitions:
       here
     mode: '0600'
     owner: "{{ matrix_user_name }}"
-    group: "{{ matrix_user_groupname }}"
+    group: "{{ matrix_group_name }}"
 ```
 
 Configuring [GCM/FCM](https://firebase.google.com/docs/cloud-messaging/) is easier, as it only requires that you provide some config values.