[w3c/webauthn] 7642dd: Editorial revisions

  Branch: refs/heads/vgb-u2f-attestation
  Home:   https://github.com/w3c/webauthn
  Commit: 7642dd6d15e80e89cd0a38ca35b889b42845f840
      https://github.com/w3c/webauthn/commit/7642dd6d15e80e89cd0a38ca35b889b42845f840
  Author: Vijay Bharadwaj <vijaybh@users.noreply.github.com>
  Date:   2017-02-08 (Wed, 08 Feb 2017)

  Changed paths:
    M index.bs

  Log Message:
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  Editorial revisions

Thanks to @equalsJeffH for the detailed review.

Remaining items: Refactor signature format section, possibly rename
fields for brevity, add CDDL/ABNF, fix U2F attestation issues.