[manifest] Fixed up titles and dfns (#397)

We made some fixes to how Respec handles `<dfn>`s. This fixes all the resulting warnings. Only markup changes in this commit. 
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/manifest/pull/397

-- Commit Summary --

  * Fixed up titles and dfns

-- File Changes --

    M index.html (457)

-- Patch Links --

https://github.com/w3c/manifest/pull/397.patch
https://github.com/w3c/manifest/pull/397.diff

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https://github.com/w3c/manifest/pull/397