Set isShutDown flag to true on module shutDown

isDestroyed flag was set to true instead of isShutDown. 
Might mislead a user.

Source/BansheeUtility/Include/BsModule.h

@@ -104,7 +104,7 @@ namespace bs
 			((Module*)_instance())->onShutDown();
 
 			bs_delete(_instance());
-			isDestroyed() = true;
+			isShutDown() = true;
 		}
 
 		/** Query if the module has been started. */
@@ -170,4 +170,4 @@ namespace bs
 	};
 
 	/** @} */
-}
+}