fix typo
Avoid vague "previous versions", and state TLS versions explicitly

Several places in the spec referred to "previous versions..." but were
imported from TLS 1.2's spec -- we should mention which versions
explicitly, to avoid ambiguity about whether these refer to TLS 1.2 or
TLS 1.3.
Finish/cleanup removal of RSA key exchange

This leaves ClientKeyExchange looking a little silly, with its
apparently unnecessary select{}, but that's needed to provide the
framework for ECDHE and other key exchange mechanisms.
Merge pull request #48 from dkg/master

fix typo