ipv4: update RFC number of IPv6 specification in docs

Change-Id: Ia3dbad890e3574b34370034fd4cb4bf96323bab3
Reviewed-on: https://go-review.googlesource.com/49790
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
1 file changed