I don't think it's possible to change the system message that is presented for the In App Purchase.
I added an In App Purchase to one of my apps recently and didn't come across the option/ability to tailor that message.
What's more, looking at some of the apps that I've downloaded that have In App Purchases, they all have the same, default wording.
I assume that Apple have added the word, "one", to make it explicit that the user is buying a quantity of one, of whatever the IAP is. Allowing developers to change this text would open up the possibility for ambiguity in the purchase process, potentially miss-selling, whether deliberate or accidental...
Hope that helps,
Gavin