A-Frame plugin: tweaks to the docs

docs/api/plugin-aframe.md

 ...
 
 <!-- External assets -->
-<a-assets>
-    <video id="my-video" hidden muted loop playsinline autoplay>
-        <source src="my-video.webm" type="video/webm" />
-        <source src="my-video.mp4" type="video/mp4" />
-    </video>
-</a-assets>
+<video id="my-video" hidden muted loop playsinline autoplay>
+    <source src="my-video.webm" type="video/webm" />
+    <source src="my-video.mp4" type="video/mp4" />
+</video>
 ```
 
 ### ar-canvas-source
 
     tick()
     {
-        const ar = this.ar;
+        const scene = this.el.sceneEl;
+        const ar = scene.systems.ar;
         const session = ar.session;
 
         // ...
 
     // ...
 
-    get ar()
-    {
-        return this.el.sceneEl.systems.ar;
-    }
-
 });
 ```
 
 
 **Details**
 
-* `ar: object`. A reference to the [ar](#ar) system.
+* `ar: object`. A reference to the [ar](#ar) system.