revert s/_/-/ changes for ryu.conf

cfg.py automatically replaces '-' in option names to '_' for config files.
so s/_/-/ in this file was a mistake.  revert the commit.

Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>