ipv6: fix RFC5722 comment

RFC5722 Section 4 was amended by Errata 3089

Our implementation did the right thing anyway...

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Acked-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>