removing getComputedAttribute from CannonDriver.md

getComputedAttribute deprecated in favor of getAttribute since AFrame 0.5

https://github.com/aframevr/aframe/blob/6aea9e7bddaa5f7f1188464ed8d9cedfcb8c41e9/CHANGELOG.md#deprecations-8

CannonDriver.md

@@ -219,7 +219,7 @@ In A-Frame, each entity's `CANNON.Body` instance is exposed on the `el.body` pro
 var el = sceneEl.querySelector('#nyan');
 el.body.applyImpulse(
   /* impulse */        new CANNON.Vec3(0, 1, -1),
-  /* world position */ new CANNON.Vec3().copy(el.getComputedAttribute('position'))
+  /* world position */ new CANNON.Vec3().copy(el.getAttribute('position'))
 );
 ```