[w3c/manifest] Editorial: clean up ExternalApplicationResource (#914)

This change (choose one):

* Makes only editorial changes (only changes informative sections, or
  changes normative sections without changing behavior)
Commit message:
Remove some redundancy. Make section a bit more consistent with the rest of the spec.


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<a href="https://pr-preview.s3.amazonaws.com/w3c/manifest/pull/914.html" title="Last updated on Jul 7, 2020, 8:50 AM UTC (e356535)">Preview</a> | <a href="https://pr-preview.s3.amazonaws.com/w3c/manifest/914/4ab46a2...e356535.html" title="Last updated on Jul 7, 2020, 8:50 AM UTC (e356535)">Diff</a>
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/manifest/pull/914

-- Commit Summary --

  * Editorial: clean up ExternalApplicationResource

-- File Changes --

    M index.html (72)

-- Patch Links --

https://github.com/w3c/manifest/pull/914.patch
https://github.com/w3c/manifest/pull/914.diff

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