ryu.conf: obsoleted argument used in sample configuration

ofp_listen_port was obsoleted and users should use ofp_tcp_listen_port
(or ofp_ssl_listen_port if ssl enabled) instead.

Signed-off-by: Wei-Li Tang <alextwl@xinguard.com>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>