Updates a coupon.
Only the properties passed in the specification object will be updated. All other properties will remain the same.
To remove a value from the coupon, pass its corresponding property with a value of null.
When updating a coupon, you cannot change the coupon's type. For example, if the coupon's type is moneyOffAmount,
you cannot change it to fixedPriceAmount. You can update the coupon type's value. For example, you can change the
value of moneyOffAmount from 5 to 10.
The coupon scope defines the items a coupon applies to. A coupon can apply to all items in a specific Wix application,
a group within the application, or a single item within a group.
See valid scope values.
You can only call this method when authenticated as a Wix app or Wix user identity.