Fix typo in previous commit

This actually had cascading effects on the generated IDs.

Author: domenic

source

@@ -80417,7 +80417,7 @@ interface <dfn>History</dfn> {
 
   <div w-nodev>
 
-  <p>The <dfn><code data-x="dom-history-index">length</code></dfn> attribute of the
+  <p>The <dfn><code data-x="dom-history-index">index</code></dfn> attribute of the
   <code>History</code> interface, on getting, must return the index of the <span>current entry of
   the joint session history</span> within the <span>top-level browsing context</span>'s <span>joint
   session history</span>. If this <code>History</code> object is associated with a