Added default gcmEndpoint value

Author: stankovski

specification/notificationhubs/resource-manager/Microsoft.NotificationHubs/stable/2017-04-01/notificationhubs.json

@@ -1972,7 +1972,7 @@
       "properties": {
         "gcmEndpoint": {
           "type": "string",
-          "description": "The GCM endpoint."
+          "description": "The FCM legacy endpoint. Default value is 'https://fcm.googleapis.com/fcm/send'"
         },
         "googleApiKey": {
           "type": "string",