[FIX] point_of_sale: discount product with tax

When you are using a discount product with a tax, you get a traceback
when trying to add the discount. It is caused by a typo and is fixed
here.

closes odoo/odoo#63550

X-original-commit: b389dcabf49fd05c5239d68fd26ef4239963a7e5
Signed-off-by: pimodoo <pimodoo@users.noreply.github.com>

addons/pos_discount/static/src/js/DiscountButton.js

@@ -49,7 +49,7 @@ odoo.define('pos_discount.DiscountButton', function(require) {
             // We add the price as manually set to avoid recomputation when changing customer.
             var base_to_discount = order.get_total_without_tax();
             if (product.taxes_id.length){
-                var first_tax = this.pos.taxes_by_id[product.taxes_id[0]];
+                var first_tax = this.env.pos.taxes_by_id[product.taxes_id[0]];
                 if (first_tax.price_include) {
                     base_to_discount = order.get_total_with_tax();
                 }