[FIX] sale_margin: correct computation of margin in groupby in the SO

For now, the sale margin is incorrectly computed in group by in the sales order.

This was done in the sales order lines in abdb85fb

opw-3180556

closes odoo/odoo#119333

Signed-off-by: Morgane Demesmaeker <edm@odoo.com>

addons/sale_margin/models/sale_order.py

@@ -60,7 +60,9 @@ class SaleOrder(models.Model):
     _inherit = "sale.order"
 
     margin = fields.Monetary("Margin", compute='_compute_margin', store=True)
-    margin_percent = fields.Float("Margin (%)", compute='_compute_margin', store=True)
+    margin_percent = fields.Float(
+        "Margin (%)", compute='_compute_margin', store=True, group_operator='avg'
+    )
 
     @api.depends('order_line.margin', 'amount_untaxed')
     def _compute_margin(self):