docs: tproxy: ignore non-transparent sockets in iptables

The iptables example was added in commit d2f26037 (netfilter: Add
documentation for tproxy, 2008-10-08), but xt_socket 'transparent'
option was added in commit a31e1ffd (netfilter: xt_socket: added new
revision of the 'socket' match supporting flags, 2009-06-09).

Now add the 'transparent' option to the iptables example to ignore
non-transparent sockets, which is also consistent with the nft example.

Signed-off-by: 谢致邦 (XIE Zhibang) <Yeking@Red54.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>