[FIX] l10n_fr_pos_cert: decrease quantity

- Add a product => 1 unit is added
- Try to change the quantity to 1.2

The 'Decrease Quantity' pop-up appears when pressing 1.

opw-2263913

closes odoo/odoo#52245

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