rest_firewall: refine the text
rest_firewall: refine the text

Signed-off-by: Yuichi Ito <7a83edf5b53f0b21d4182a3da7518164ce03dc3a@gmail.com>

iwaseyusuke/ryu-book,iwaseyusuke/ryu-book,osrg/ryu-book,osrg/ryu-book