tor-spec: fix ambitious formulation in VERSIONS

This commit fixes an ambitious formulation within the definition of the VERSIONS cells. It says, that a VERSIONS cell with an odd number of bytes is invalid. This statement is not true, because the CircID (2 bytes for VERSIONS cells), Command (1 byte) and Length (2 byte) make up 5 bytes, which is an odd number. Adding an odd number to an even number of bytes (the payload in this case) always results in an odd number.