added event ref to spawn prefab

UOP1_Project/Assets/Prefabs/GameplayEssentials/SpawnSystem.prefab

     type: 2}
   _spawnLocations:
   - {fileID: 2125786286893897154}
+  _OnSceneReady: {fileID: 11400000, guid: b729e40fc41dd8b4ea7aaf5c857f7186, type: 2}
 --- !u!1 &2125786286893897213
 GameObject:
   m_ObjectHideFlags: 0