crypto/tls: Update reference to the TLS 1.2 RFC.
crypto/tls: Update reference to the TLS 1.2 RFC.

Ticket 13740047 updated the documented TLS version to 1.2.
This also updates the RFC refered to.

R=golang-dev
CC=golang-dev, rsc
https://codereview.appspot.com/14029043

Committer: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

pombredanne/go-deleteme,tv42/old-go,apparentlymart/golang,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,apparentlymart/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,apparentlymart/golang,tv42/old-go,tv42/old-go,tv42/old-go