[w3c/payment-handler] fix: replace deprecated `wg` with `group` (#385)

This fixes the CI failure.


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<a href="https://pr-preview.s3.amazonaws.com/saschanaz/payment-handler/pull/385.html" title="Last updated on Apr 1, 2021, 5:09 PM UTC (2d0b3de)">Preview</a> | <a href="https://pr-preview.s3.amazonaws.com/w3c/payment-handler/385/4b4525f...saschanaz:2d0b3de.html" title="Last updated on Apr 1, 2021, 5:09 PM UTC (2d0b3de)">Diff</a>
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/payment-handler/pull/385

-- Commit Summary --

  * fix: replace deprecated `wg` with `group`

-- File Changes --

    M index.html (6)

-- Patch Links --

https://github.com/w3c/payment-handler/pull/385.patch
https://github.com/w3c/payment-handler/pull/385.diff

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