doc: fix seg6_flowlabel path

This sysctl is not per interface; it's global per netns.

Fixes: 292ecd9f ("doc: move seg6_flowlabel to seg6-sysctl.rst")
Reported-by: Philippe Guibert <philippe.guibert@6wind.com>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>