Be more explicit about the certificate format in `tor-spec.txt`
Section 4.2. (“CERTS cells”) of`tor-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
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