fix a typo in a comment

 * contents themselves may be another Ed25519 key, a digest of a

 * RSA key, or some other material.

 *

 * In this module there is also support for a crooss-certification of

 * In this module there is also support for a cross-certification of

 * Ed25519 identities using (older) RSA1024 identities.

 *

 * Tor uses other types of certificate too, beyond those described in this