[FIX] stock: incorrect field

The field is now `quantity`. This wasn't spotted before because the
method is not used anywhere.

opw-2008491

closes odoo/odoo#34067

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

addons/stock/models/stock_quant.py

@@ -490,5 +490,5 @@ class QuantPackage(models.Model):
         for quant in self._get_contained_quants():
             if quant.product_id not in res:
                 res[quant.product_id] = 0
-            res[quant.product_id] += quant.qty
+            res[quant.product_id] += quant.quantity
         return res