[FIX] product_pricelist_total_margin: Change field string to avoid redundancy

- Change margin_type string from 'Margin Type' to 'Calculation Method'
- Fixes pylint warning W8113 (attribute-string-redundant)

product_pricelist_total_margin/models/product_pricelist_item.py

@@ -28,7 +28,7 @@ class ProductPricelistItem(models.Model):
             ("markup", "Markup (on cost)"),
             ("margin", "Commercial Margin (on PVP)"),
         ],
-        string="Margin Type",
+        string="Calculation Method",
         default="markup",
         help="Type of margin calculation:\n"
         "- Markup: PVP = Cost × (1 + markup%). Margin is calculated on cost.\n"