fix method to get attribute names

Author: arottier

dom.bs

@@ -3627,7 +3627,7 @@ run these steps:
  {{MutationObserverInit/elementHasAttributeNameFilter}} is present, then:
 
  <ol>
-  <li><p>Let <var>nodeAttributesList</var> be <var>nodeAttributesList</var> of the node's attributes, "<code>node.attributes.keys</code>".
+  <li><p>Let <var>nodeAttributesList</var> be <var>nodeAttributesList</var> of the node's attributes, "<code>node.getAttributeNames()</code>".
 
   <li><p>Let <var>filteredAddedNodes</var> be an empty <a for=/>list</a>.