[w3c/webauthn] bc195a: update token binding citation (#1087)

  Branch: refs/heads/master
  Home:   https://github.com/w3c/webauthn
  Commit: bc195aad084d73e570c581b12f866caff60ec222
      https://github.com/w3c/webauthn/commit/bc195aad084d73e570c581b12f866caff60ec222
  Author: =JeffH <mobileuser@kingsmountain.com>
  Date:   2018-10-22 (Mon, 22 Oct 2018)

  Log Message:
  -----------
  update token binding citation (#1087)

* update token binding citation

* update tokbind ref to published RFC8471



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