theme: replace <div> with <header> for #menu-bar

This replacement makes mdbook more semantically
correct and should remove "Light; Rust; Coal; Navy; Ayu. Title"
text from mdbook preview in Google search.

src/theme/index.hbs

@@ -144,7 +144,7 @@
             <div class="page">
                 {{> header}}
                 <div id="menu-bar-hover-placeholder"></div>
-                <div id="menu-bar" class="menu-bar sticky">
+                <header id="menu-bar" class="menu-bar sticky">
                     <div class="left-buttons">
                         <label id="sidebar-toggle" class="icon-button" for="sidebar-toggle-anchor" title="Toggle Table of Contents" aria-label="Toggle Table of Contents" aria-controls="sidebar">
                             <i class="fa fa-bars"></i>
@@ -186,7 +186,7 @@
                         {{/if}}
 
                     </div>
-                </div>
+                </header>
 
                 {{#if search_enabled}}
                 <div id="search-wrapper" class="hidden">