Fix the payment link creation script

Fixes #131 (closed)

This doesn't fixed the deployed state in Stripe so I manually fixed that with stripe-cli.

stripe --api-key $(cat ... ) products list
stripe --api-key $(cat ... ) products update prod_MjBCW0d4kZUz6f --description "30 GB-months of Private.Storage storage × time"
stripe --api-key $(cat ... ) products update prod_MjBCW0d4kZUz6f --name "30 GB-months"

This fixed the product but didn't fix every mention of "GiB" on the checkout page so I also ran:

stripe --api-key $(cat ... ) payment_links update plink_1LziqhA9OAm23rYOB1vnGcQq -d 'line_items[0][id]=li_MjBDo1OLF0GeeA' -d 'line_items[0][quantity]=1'

where li_MjBDo1OLF0GeeA is the identifier of the line item on the payment link for the one and only product we're selling. The quantity value given is the same as the original value but running this seems to bump something inside Stripe and fix the last "GiB" reference.