Update tor-spec for ed25519 link authentication keys

tor-spec.txt says that the ed25519 link authentication key isn't used:

   These are Ed25519 keys:

    - A long-term "master identity" key.  This key never
      changes; it is used only to sign the "signing" key below.  It may be
      kept offline.
    - A medium-term "signing" key.  This key is signed by the master identity
      key, and must be kept online.  A new one should be generated
      periodically.
    - A short-term "link authentication" key.  Not yet used.