Update the config documentation to note the value `2` as an acceptable
value for the protocol.version config.
Signed-off-by: Brandon Williams <bmwill@google.com> Signed-off-by: Josh Steadmon <steadmon@google.com> Reviewed-by: Jonathan Nieder <jrnieder@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>