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://golang.org/cl/14029043

src/pkg/crypto/tls/tls.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// Package tls partially implements TLS 1.2, as specified in RFC 4346.
+// Package tls partially implements TLS 1.2, as specified in RFC 5246.
 package tls
 
 import (