Fix menu button accessibility issue on mobile

Author: c-w

src/index.html

@@ -26,7 +26,7 @@
 
       <div class="navbar-header">
 
-        <button type="button" class="navbar-toggle" id="navbar-dropdown" data-toggle="collapse" data-target=".navbar-collapse">
+        <button type="button" class="navbar-toggle" id="navbar-dropdown" data-toggle="collapse" data-target=".navbar-collapse" aria-label="Navigation menu">
           <span class="glyphicon glyphicon-align-justify" id="menu-icon"></span>
         </button>
         <a class="navbar-brand" href="#home-page">