Update dir-spec with the correct Tor version when bandwidth-file-digest is merged
It currently says 0.4.0.1-alpha, but this feature didn't make it: https://gitweb.torproject.org/torspec.git/tree/dir-spec.txt#n2162
It currently says 0.4.0.1-alpha, but this feature didn't make it: https://gitweb.torproject.org/torspec.git/tree/dir-spec.txt#n2162