[manifest] Changed file extension of web manifest (closes #346) (#349)

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

  https://github.com/w3c/manifest/pull/349

-- Commit Summary --

  * Changed file extension of web manifest (closes #346)

-- File Changes --

    M index.html (2)

-- Patch Links --

https://github.com/w3c/manifest/pull/349.patch
https://github.com/w3c/manifest/pull/349.diff

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https://github.com/w3c/manifest/pull/349