move table-of-contents id to nav element

static/usage.html

@@ -50,10 +50,8 @@
             only cover the user-facing impact of features, and most of the privacy and security
             work is done under the hood.</p>
 
-            <nav>
-                <h2 id="table-of-contents">
-                    <a href="#table-of-contents">Table of contents</a>
-                </h2>
+            <nav id="table-of-contents">
+                <h2><a href="#table-of-contents">Table of contents</a></h2>
                 <ul>
                     <li><a href="#auditor">Auditor</a></li>
                     <li>