doc/man (vpn-network-options): fix foreign_option_{n} typo

doc/man (vpn-network-options): fix foreign_option_{n} typo

In 2da29362 (Improve the documentation for --dhcp-option, 2020-08-16),
`foreign_option_{n}` became plural between the first and second versions
of the patch.  Correct it.

Signed-off-by: Todd Zullinger <tmz@pobox.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20211122114104.4814-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg23217.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit aee54a47142c13a29c2b59142e944fd448a406d9)