[FIX] mrp: fix singleton error in _update_raw_move

Commit bfb0f1e2a5
did a very good job not seeing that the lines only used move[0],
and called move.field directly.
This could result, for certain BoMs, in traceback.

opw 1928677

closes odoo/odoo#30938

addons/mrp/models/mrp_production.py

@@ -455,7 +455,7 @@ class MrpProduction(models.Model):
                 move[0].with_context(do_not_unreserve=True).write({'product_uom_qty': quantity})
                 move[0]._recompute_state()
                 move[0]._action_assign()
-                move.unit_factor = quantity / move.raw_material_production_id.product_qty
+                move[0].unit_factor = quantity / move[0].raw_material_production_id.product_qty
             elif quantity < 0:  # Do not remove 0 lines
                 if move[0].quantity_done > 0:
                     raise UserError(_('Lines need to be deleted, but can not as you still have some quantities to consume in them. '))