[w3c/payment-request] Fix typo in code sample. (#907)

The code sample at https://w3c.github.io/payment-request/#example-6-constructing-a-paymentrequest has a `request.retry()` statement, but the `retry()` method is defined on the `PaymentResponse` object. So the code sample should replace `request.retry()` with `response.retry()`. Thank you, @agektmr, for pointing this out!

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