[w3c/payment-request] Fix grammatical error (#756)

Just a small, grammatical fix.

The following tasks have been completed:

 * [x] Confirmed there are no ReSpec errors/warnings.
 * [ ] Added Web platform tests (link)
 * [ ] added MDN Docs (link)
 * [ ] added MDN [compat data](https://github.com/mdn/browser-compat-data)

Implementation commitment:

 * [ ] Safari (link to issue)
 * [ ] Chrome (link to issue)
 * [ ] Firefox (link to issue)
 * [ ] Edge (public signal)

Impact on Payment Handler spec?

You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/payment-request/pull/756

-- Commit Summary --

  * Fix grammatical error

-- File Changes --

    M index.html (3)

-- Patch Links --

https://github.com/w3c/payment-request/pull/756.patch
https://github.com/w3c/payment-request/pull/756.diff

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