extensions: manpages: cleanup hyphens

Cleanup, scape hyphens so they are not interpreted by the manpage generator.

Arturo says:
 This patch is forwarded from the iptables Debian package, where it has been
 around for many years now.

Signed-off-by: Laurence J. Lane <ljlane@debian.org>
Signed-off-by: Arturo Borrero Gonzalez <arturo@netfilter.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>