ofproto: fixed comments of ofp_capabilities.

Signed-off-by: Wataru ISHIDA <ishida.wataru@lab.ntt.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Merge remote-tracking branch 'upstream/master' into dpi
dpi: prototype rest-dpi