Add class if only 1 item left in stock

functions.php

/**
 * Add class if only 1 item left in stock
 * Author: MarioMarkovic.com
 */
function filter_get_stock_html( $html, $product ) {
    // Low stock quantity amount
    $low_stock_qty = 1;

    $availability = $product->get_availability();

    if ( ! empty( $availability['availability'] ) ) {
        $class = esc_attr( $availability['class'] );
        $avail_text = wp_kses_post( $availability['availability'] );
        $stock_qty = $product->get_stock_quantity();

        if( $stock_qty == $low_stock_qty ){
            $class .= ' one-in-stock';
            $avail_text = __('Samo še 1 na zalogi', 'woocommerce');
        }
        ob_start();

        // Make your changes below
        ?>
        <p class="stock <?php echo $class; ?>"><?php echo $avail_text; ?></p>
        <?php

        $html = ob_get_clean();
    }
    return $html;
}
add_filter( 'woocommerce_get_stock_html', 'filter_get_stock_html', 10, 2 );