[FIX] sale: visible cost price in sale order line

The Cost of the product(purchase_price) must be shown in the wizard opened
when changing a SO line.

opw:690180

addons/sale_margin/views/sale_margin_view.xml

@@ -20,7 +20,7 @@
         <field name="inherit_id" ref="sale.view_order_form"/>
         <field name="arch" type="xml">
             <xpath expr="//field[@name='order_line']/form//field[@name='price_unit']" position="after">
-                <field name="purchase_price" groups="base.group_user" invisible="True"/>
+                <field name="purchase_price" groups="base.group_user"/>
             </xpath>
         </field>
     </record>