[push-api] Fixing definition of push message (#126)

The definition here was a little strange, probably as a legacy of the original "no data" API.

It also contained unnecessary normative language.
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/push-api/pull/126

-- Commit Summary --

  * Fixing definition of push message

-- File Changes --

    M index.html (10)

-- Patch Links --

https://github.com/w3c/push-api/pull/126.patch
https://github.com/w3c/push-api/pull/126.diff

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/push-api/pull/126