Be more explicit about the certificate format in `tor-spec.txt`

Section 4.2. (“CERTS cells”) oftor-spec.txt contains the phrase:

The certificate format for the above certificate types is X509.

However, X.509 certificates can be uncoded in a number of different formats, DER being the one that's used with Tor.

See the attached patch to improve the sentence like this:

The certificate format for the above certificate types is DER encoded X509.

Trac:
Username: herzi