Add item ONLY to the primairy navigation

i did find my answer here : http://www.wpbeginner.com/wp-themes/how-to-add-custom-items-to-specific-wordpress-menus/

add_filter( 'wp_nav_menu_items', 'your_custom_menu_item', 10, 2 );
function your_custom_menu_item ( $items, $args ) {
    if (is_single() && $args->theme_location == 'primary') {
        $items .= '<li>Show whatever</li>';
    }
    return $items;
}