Change example

docs/api/plugin-aframe.md

 
     tick()
     {
-        const ar = this.el.sceneEl.systems.ar;
+        const ar = this.ar;
         const session = ar.session;
 
         // ...
 
     // ...
 
+    get ar()
+    {
+        return this.el.sceneEl.systems.ar;
+    }
+
 });
 ```