Cart should say Expiration and not Expiry

[ajgoldenwings]

Cart Expiration Date says Expiry http://screencast.com/t/M0fbYqvJ

[targeter21]

The terms are technically interchangeable. You can either say 'Expiry Date' vs 'Expiration'. That said for the purpose of this demo app, 'Expiration' is probably less distracting. I made PR https://github.com/Polymer/shop/pull/115 for this if the polymer shop owners wish to go that route.