fix bug

Documentation/apm.md

 
 ### Optional: Using APM Game SDK
 In the initialization method at the beginning of the game, call the startGamePlugin() method of APMS to enable the SDK
+```
 void Start()
 {
     // In order to start APMS Game Plugin, you need to call this method
     // ...
 }
-
+```
+```
 void OnDestroy() {
     // In order to start APMS Game Plugin, you need to call this method
     GameAPM.getInstance().stop();
+```
+```
 public void scenceLoadToGame()
 {
 	// start record the loading scene time
 	// stop record the loading scene time
 	GameAPM.getInstance().stopLoadingScene(scene);
 }
+```
+
 Prerequisites：
 1. Your application has enabled the APM service.
 2. Your application has integrated the APM Game SDK and runs on the device.