added additional words about Mastercard's other numbers, to make it clear that students should only be using 51-55 and not worrying about other sample ranges that are shown on the linked PayPal page. (This change was originally made for the 2018/x spec but didn't carry over to the 2019/x version: https://github.com/cs50/docs/issues/198)
added additional words about Mastercard's other numbers, to make it clear that students should only be using 51-55 and not worrying about other sample ranges that are shown on the linked PayPal page. (This change was originally made for the 2018/x spec but didn't carry over to the 2019/x version: https://github.com/cs50/docs/issues/198)