Document the subscription_id property for ProjectRef

Author: pjcdawkins

src/Model/Ref/ProjectRef.php

@@ -7,6 +7,7 @@
 /**
  * @property-read string $id
  * @property-read string $organization_id
+ * @property-read string $subscription_id
  * @property-read string $region
  * @property-read string $title
  * @property-read string $created_at