add missed grw-hljs-container-for-demo element

src/server/views/admin/customize.html

@@ -19,6 +19,9 @@
 {% block content_main %}
 <div class="content-main admin-customize row">
   {% parent %}
+  <div id="grw-hljs-container-for-demo">
+    {{ cdnHighlightJsStyleTag(getConfig('crowi', 'customize:highlightJsStyle')) }}
+  </div>
   <div class="col-lg-9" id="admin-customize"></div>
 </div>
 {% endblock content_main %}