Fix variable reference in "get the history object length and index"

Closes whatwg#8619.

Author: jakearchibald

source

@@ -92478,7 +92478,7 @@ location.href = '#foo';</code></pre>
    </li>
 
    <li><p>Let <var>scriptHistoryIndex</var> be the index of <var>step</var> in
-   <var>sessionSteps</var>.</p></li>
+   <var>steps</var>.</p></li>
 
    <li><p>Return (<var>scriptHistoryLength</var>, <var>scriptHistoryIndex</var>).</p></li>
   </ol>