crypto/tls: Update references to draft-ietf-tls-downgrade-scsv-00.

It's RFC 7507 now.

Change-Id: Iccd6c65f9d4b1f4d17ee068dee4576a512ba8405
Reviewed-on: https://go-review.googlesource.com/21154Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/crypto/tls/cipher_suites.go

@@ -286,6 +286,6 @@ const (
 
 	// TLS_FALLBACK_SCSV isn't a standard cipher suite but an indicator
 	// that the client is doing version fallback. See
-	// https://tools.ietf.org/html/draft-ietf-tls-downgrade-scsv-00.
+	// https://tools.ietf.org/html/rfc7507.
 	TLS_FALLBACK_SCSV uint16 = 0x5600
 )

src/crypto/tls/handshake_server.go

@@ -246,7 +246,7 @@ Curves:
 		return false, errors.New("tls: no cipher suite supported by both client and server")
 	}
 
-	// See https://tools.ietf.org/html/draft-ietf-tls-downgrade-scsv-00.
+	// See https://tools.ietf.org/html/rfc7507.
 	for _, id := range hs.clientHello.cipherSuites {
 		if id == TLS_FALLBACK_SCSV {
 			// The client is doing a fallback connection.