[FIX] payment: portal payment use newly generated reference

- When creating a transaction on the payment page of the portal, the
  transaction reference returned was incorrect.
  This commit return the newly created transaction's reference.

addons/payment/controllers/portal.py

@@ -118,7 +118,7 @@ class WebsitePayment(http.Controller):
             'partner_id': partner_id,
         }
 
-        return acquirer.sudo().render(reference, float(amount), int(currency_id), values=render_values)
+        return acquirer.sudo().render(tx.reference, float(amount), int(currency_id), values=render_values)
 
     @http.route(['/website_payment/token/<string:reference>/<string:amount>/<string:currency_id>',
                 '/website_payment/token/v2/<string:amount>/<string:currency_id>/<path:reference>'], type='http', auth='public', website=True)