[FIX] stock: fix wrong domain

Commit 6c4447ef59 set a new domain on picking_type_id of stock rule but
it was wrongly written. This commit make it syntactically correct.

closes odoo/odoo#43009

Task: 2169836
Signed-off-by: Simon Lejeune (sle) <sle@openerp.com>

addons/stock/models/stock_rule.py

@@ -57,7 +57,7 @@ class StockRule(models.Model):
     picking_type_id = fields.Many2one(
         'stock.picking.type', 'Operation Type',
         required=True, check_company=True,
-        domain=[('code', '?=', 'picking_type_code_domain')])
+        domain="[('code', '=?', picking_type_code_domain)]")
     picking_type_code_domain = fields.Char(compute='_compute_picking_type_code_domain')
     delay = fields.Integer('Delay', default=0, help="The expected date of the created transfer will be computed based on this delay.")
     partner_address_id = fields.Many2one(