Fix JSON formatting in docs

Using-the-API/Push-notifications.md

@@ -11,7 +11,7 @@ When the app obtains/refreshes a registration ID from Firebase, it needs to send
 The push notifications will be triggered by the notifications of the type you can normally find in `/api/v1/notifications`. However, the push notifications will not contain any inline content. They will contain JSON data of this format ("12" is an example value):
 
 ```json
-{ notification_id: 12 }
+{ "notification_id": 12 }
 ```
 
 Your app can then retrieve the actual content of the notification from the `/api/v1/notifications/12` API endpoint.