Nit/trivial: Fix Anonymous functions Example # 4 cart "add" method to support adding quantity to an existing product

[willy-b]

Nit/trivial - feel free to ignore or accept, figured I'd share in case you are interested to take the update.

At https://web.archive.org/web/20261014211813/https://www.php.net/manual/en/functions.anonymous.php#:~:text=add($product,%20$quantity)

the add method of the shopping cart example was written:

public function add($product, $quantity)
{
    $this->products[$product] = $quantity;
}

The implementation does not match the name, and is more like "set",
as if one called add twice for the same product it would not increment the quantity but replace with the value it was last called with, e.g.:

$my_cart->add('butter', 1);
$my_cart->add('butter', 1);

would result in a quantity of 1 for butter instead of 2.

This commit fixes that to avoid distraction by a strange add method from the point of the example which is the getTotal method.