Update UIMenuManager.cs

Found a small bug, which was causing a NullReferenceException

Bug ticket:
https://github.com/UnityTechnologies/open-project-1/issues/478

Forum Thread:
https://forum.unity.com/threads/bug-uimenumanager-throws-nullreferenceexception-on-game-exit.1142410/#post-7337182

UOP1_Project/Assets/Scripts/UI/UIMenuManager.cs

 		if (quitConfirmed)
 		{
 			Application.Quit();
-			_onGameExitEvent.OnEventRaised();
+			_onGameExitEvent.RaiseEvent();
 		}
 		_mainMenuPanel.SetMenuScreen(_hasSaveData);