[w3c/webauthn] dda3f4: Update index.bs

  Branch: refs/heads/ve7jtb-fix-1400-Fix-IANA-registry-for-extensions
  Home:   https://github.com/w3c/webauthn
  Commit: dda3f4279a46182f99c29f29f703533f4c74e849
      https://github.com/w3c/webauthn/commit/dda3f4279a46182f99c29f29f703533f4c74e849
  Author: John Bradley <ve7jtb@ve7jtb.com>
  Date:   2020-04-21 (Tue, 21 Apr 2020)

  Changed paths:
    M index.bs

  Log Message:
  -----------
  Update index.bs

Fixes #1400 adds IANA registration for appidExclude and removes allready registerd extensions.  Changes wording from initial registrations to additional registrations.