Editorial: Align with Web IDL specification (#64)

Author: autokagami

index.html

@@ -306,7 +306,7 @@ <h2>Extensibility</h2>
   <pre class="idl">
     partial interface Element {
       [CEReactions] attribute [LegacyNullToEmptyString] DOMString outerHTML;
-      [CEReactions] void insertAdjacentHTML(DOMString position, DOMString text);
+      [CEReactions] undefined insertAdjacentHTML(DOMString position, DOMString text);
     };
   </pre>