[css-animations-2] Update code sample

Effects no longer need to be cloned in order to be mutated.

Author: birtles

css-animations-2/Overview.bs

@@ -131,13 +131,11 @@ elem.style.animation = 'move 1s';
 let animation = elem.getAnimations()[0];
 // animation.effect.target == elem == animation's owning element
 
-let mutableEffect = animation.effect.clone();
-animation.effect = mutableEffect;
 animation.effect.target = elem2;
 // animation.effect.target == elem2 != animation's owning element
 
 animation.effect = null;
-// animation.effect.target is undefined != animation's owning element
+// animation.effect?.target is undefined != animation's owning element
 </pre>
 
 </div>