[FIX] point-of-sale: discount line on ticket

Before this fix an extra line with the price without discount will be
printed even if there is no discount on the product.

After this fix the line will only be printed if there is a discount.

The error occured due to the use of t-else instead of t-elif.

closes odoo/odoo#32989

Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>