[css-paint-api-1] Align with Web IDL specification (#1001)

Author: autokagami

css-paint-api/Overview.bs

@@ -90,7 +90,7 @@ identical to the Window.{{Window/devicePixelRatio}} property.
 <pre class='idl'>
 [Global=(Worklet,PaintWorklet),Exposed=PaintWorklet]
 interface PaintWorkletGlobalScope : WorkletGlobalScope {
-    void registerPaint(DOMString name, VoidFunction paintCtor);
+    undefined registerPaint(DOMString name, VoidFunction paintCtor);
     readonly attribute unrestricted double devicePixelRatio;
 };
 </pre>