[mqtt_client]: Fix docstring in subscribe_group

Fix param in the doc string of subscribe_group to use proper name.

Trivial fix

Author: flavio-fernandes

Adafruit_IO/mqtt_client.py

@@ -219,7 +219,7 @@ def subscribe_group(self, group_id, qos=0):
       """Subscribe to changes on the specified group. When the group is updated
       the on_message function will be called with the group_id and the new value.
 
-      :param str feed_id: The key of the feed to subscribe to.
+      :param str group_id: The id of the group to subscribe to.
       :param int qos: The QoS to use when subscribing. Defaults to 0.
 
       """