ipv6: update RFC number of IPv6 specification in docs

Change-Id: I38980c8463b7806c129024967713125acf663b82
Reviewed-on: https://go-review.googlesource.com/49791
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
2 files changed