bridge-8.x-1.x-dev/templates/navigation/burger.html.twig

<button type="button" id="navbar-toggle" class="navbar-toggle collapsed" data-toggle="collapse" data-target="#main-navigation" aria-expanded="false">
        <span class="sr-only">Toggle navigation</span>
        <span class="icon-bar"></span>
        <span class="icon-bar"></span>
        <span class="icon-bar"></span>
</button>