Fix /v3/info/usage_summary docu

* "service_instances" contains only managed services, not user-provided services

Author: jochenehret

docs/v3/source/includes/resources/info/_get_usage_summary.md.erb

@@ -32,7 +32,7 @@ This endpoint retrieves a high-level summary of usage across the entire Cloud Fo
 | **started_instances** | _integer_ | Total number of process instances in the `STARTED` state                                                 |
 | **memory_in_mb**      | _integer_ | Sum of memory usage of all tasks in the `RUNNING` state and all process instances in the `STARTED` state |
 | **routes**            | _integer_ | Total number of routes                                                                                   |
-| **service_instances** | _integer_ | Total number of service instances                                                                        |
+| **service_instances** | _integer_ | Total number of managed service instances                                                                |
 | **reserved_ports**    | _integer_ | Total number of reserved ports                                                                           |
 | **domains**           | _integer_ | Total number of private domains                                                                          |
 | **per_app_tasks**     | _integer_ | Total number of running tasks                                                                            |