index: rename from dependencies

Specification documents usually refer to their dependency list as
an index.  WebDriver should do the same.

Author: andreastt

index.html

@@ -103,7 +103,7 @@ <h2>Conformance</h2>
 rather than performance, in mind.
 
 <section>
-<h3>Dependencies</h3>
+<h3>Index</h3>
 
 <p>This specification relies on several other underlying specifications.
 
@@ -670,7 +670,7 @@ <h3>Dependencies</h3>
       <!-- XPathException --><li><a href="https://www.w3.org/TR/DOM-Level-3-XPath/xpath.html#XPathException"><dfn><code>XPathException</code></dfn></a>
     </ul>
 </dl>
-</section> <!-- /Dependencies -->
+</section> <!-- /Index -->
 </section> <!-- /Conformance -->
 
 <section class=informative>