Fix man page and OSCP script: tls_serial_{n} is decimal

Commit 7d5e26cbb53 fixed extracting serial but did not change the format,
which always has been decimal. This patch fixes the manpage and
OSCP.sh script to conform with the implementation.
Acked-by: James Yonan <james@openvpn.net>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1396001222-5033-1-git-send-email-arne@rfc2549.org>
URL: http://article.gmane.org/gmane.network.openvpn.devel/8409

Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 959d60789b6f0bd74296600f58f626cfa9738f78)