[w3c/manifest] Tidied up document using tidy-html5 (#1012)

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

  https://github.com/w3c/manifest/pull/1012

-- Commit Summary --

  * <a href="https://github.com/w3c/manifest/pull/1012/commits/d55b2259ac6140017e6b9a1e7d18bf9d1474695d">chore: tidy up index.html</a>

-- File Changes --

    M index.html (184)

-- Patch Links --

https://github.com/w3c/manifest/pull/1012.patch
https://github.com/w3c/manifest/pull/1012.diff

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