move table-of-contents id to nav element

static/build.html

@@ -49,10 +49,8 @@
             <p>This is a guide on building, modifying and contributing to GrapheneOS as a
             developer.</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="#building-grapheneos">Building GrapheneOS</a>