This patch describes the correct syntax of --inherit-fd.

travis-ci: success for Add/fix description of --external and --inherit-fd
CC: Andrei Vagin <avagin@virtuozzo.com>
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>