[FIX] purchase_requision: fix syntax error

payment_term.id, creates a tuple wich works if payment_term.id is set
in case payment_term.id is False this creates the tuple (False,) which
is then converted by the Odoo ORM to a account.payment.term(False, )
record set, which in term may cause weird behaviour as this value does
not eval to false in cases like `if self.payment_term_id:`

Closes odoo/odoo#28397

addons/purchase_requisition/models/purchase_requisition.py

@@ -170,7 +170,7 @@ class PurchaseOrder(models.Model):
 
         self.partner_id = partner.id
         self.fiscal_position_id = fpos.id
-        self.payment_term_id = payment_term.id,
+        self.payment_term_id = payment_term.id
         self.company_id = requisition.company_id.id
         self.currency_id = currency.id
         self.origin = requisition.name