[css-animation-worklet-1] Align with Web IDL specification (#1004)

Author: autokagami

css-animation-worklet-1/Overview.bs

@@ -163,14 +163,12 @@ The {{animationWorklet}}'s [=worklet global scope type=] is {{AnimationWorkletGl
 {{AnimationWorkletGlobalScope}} represents the global execution context of {{animationWorklet}}.
 
 <xmp class='idl'>
-
 [ Global=(Worklet,AnimationWorklet), Exposed=AnimationWorklet ]
 interface AnimationWorkletGlobalScope : WorkletGlobalScope {
-    void registerAnimator(DOMString name, AnimatorInstanceConstructor animatorCtor);
+    undefined registerAnimator(DOMString name, AnimatorInstanceConstructor animatorCtor);
 };
 
 callback AnimatorInstanceConstructor = any (any options, optional any state);
-
 </xmp>