No, the prompt was correct. Wheelwright reads, in part:
You may discard a card to gain an Action card costing as much as it or less.
Discarding a Gold means you can really gain an Action card costing up to $6. Of course, not so many Actions cards costing $6 exist, so I assume there was none in your kingdom, which means there was no discernable difference between "up to $5" and "up to $6".