Prevent visible accessibility label for nav hamberger in small window

Author: ascholerChemeketa

runestone/common/project_template/_templates/plugin_layouts/sphinx_bootstrap/layout.html

@@ -32,8 +32,8 @@
 
     <div class="navbar-header">
       <!-- .btn-navbar is used as the toggle for collapsed navbar content -->
-      <button type='button' class='navbar-toggle' data-toggle="collapse" data-target=".navbar-ex1-collapse">
-        <span class="icon-bar" aria-label="navbar toggle">toggle navbar</span>
+      <button type='button' class='navbar-toggle' data-toggle="collapse" data-target=".navbar-ex1-collapse" aria-label="navbar toggle">
+        <span class="icon-bar"></span>
         <span class="icon-bar"></span>
         <span class="icon-bar"></span>
       </button>