lib/time: update link in README; draft now an RFC

R=golang-dev, minux.ma, bradfitz
CC=golang-dev
https://golang.org/cl/7704043

lib/time/README

@@ -5,6 +5,6 @@ The IANA asserts that the database is in the public domain.
 For more information, see
 http://www.iana.org/time-zones
 ftp://ftp.iana.org/tz/code/tz-link.htm
-http://tools.ietf.org/html/draft-lear-iana-timezone-database-05
+http://tools.ietf.org/html/rfc6557
 
 To rebuild the archive, read and run update.bash.