Add missing class

Resources/views/CRUD/base_list.html.twig

@@ -156,7 +156,7 @@ file that was distributed with this source code.
                                 {% trans from 'SonataAdminBundle' %}all_elements{% endtrans %}
                             </label>
 
-                            <input type="submit" class="btn primary" value="{% trans from 'SonataAdminBundle' %}btn_batch{% endtrans %}" />
+                            <input type="submit" class="btn btn-primary" value="{% trans from 'SonataAdminBundle' %}btn_batch{% endtrans %}" />
                         {% endblock %}
                     </div>
                 {% endblock %}