Make update the only public method

I think this got lost when refactoring, but only `update` is meant
to be a public method, other than `new`.

app/services/voucher_adjustments_service.rb

@@ -32,6 +32,8 @@ class VoucherAdjustmentsService
     end
   end
 
+  private
+
   def handle_tax_excluded_from_price(voucher)
     voucher_rate = voucher.rate(@order)
     adjustment = @order.voucher_adjustments.first