Update menu icon for mobiles

Ref: phpmyadmin/website#141

Signed-off-by: William Desportes <williamdes@wdes.fr>

theme/index.html.tmpl

@@ -31,7 +31,7 @@
   <nav class="navbar navbar-expand-lg navbar-light bg-light">
     <div class="container">
       <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarSupportedContent" aria-controls="navbarSupportedContent" aria-expanded="false" aria-label="Toggle navigation">
-        <span class="navbar-toggler-icon"></span>
+        <span class="fa fa-bars"></span>
       </button>
 
       <div class="collapse navbar-collapse" id="navbarSupportedContent">