Issue #2021571 by rszrama: Preview of menu machine name is inaccurate on while adding a menu

modules/menu/menu.admin.inc

@@ -481,6 +481,7 @@ function menu_edit_menu($form, &$form_state, $type, $menu = array()) {
     '#default_value' => $menu['menu_name'],
     '#maxlength' => MENU_MAX_MENU_NAME_LENGTH_UI,
     '#description' => t('A unique name to construct the URL for the menu. It must only contain lowercase letters, numbers and hyphens.'),
+    '#field_prefix' => empty($menu['old_name']) ? 'menu-' : '',
     '#machine_name' => array(
       'exists' => 'menu_edit_menu_name_exists',
       'source' => array('title'),