fix typo in readme.md
fix typo in readme.md

According to Apple's spec, the value of version is EC_v1.
(https://developer.apple.com/library/content/documentation/PassKit/Reference/PaymentTokenJSON/PaymentTokenJSON.html)
etsy/applepay-php,etsy/applepay-php,etsy/applepay-php,etsy/applepay-php