[manifest] Remove reference to RFC5988. (#428)

As far as I know this is not implemented. Maybe someone has context about why it was added in the first place?

Closes #424.
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/manifest/pull/428

-- Commit Summary --

  * Remove reference to RFC5988.

-- File Changes --

    M index.html (4)

-- Patch Links --

https://github.com/w3c/manifest/pull/428.patch
https://github.com/w3c/manifest/pull/428.diff

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https://github.com/w3c/manifest/pull/428