Updated coupon name description

Creating a second coupon with the same name as an existing coupon will cause the API to return `409 Conflict`. Hence, the name parameter must be unique.

reference/marketing.v2.yml

@@ -943,7 +943,7 @@ components:
       properties:
         name:
           type: string
-          description: The name of the coupon.
+          description: The name of the coupon. Value must be unique.
           example: Australia Customers Discount
           maxLength: 100
         type: