Fix typo in CONTRIBUTING.md
Fix typo in CONTRIBUTING.md

Signed-off-by: zhoulin xie <1307e474708ef2b3861f4fdec84b75b2c3a221f4@daocloud.io>

joshix/rkt,joshix/rkt,coreos/rkt,kinvolk/rkt,rkt/rkt,rkt/rkt,coreos/rkt,coreos/rkt,rkt/rkt,kinvolk/rkt,joshix/rkt,rkt/rkt,kinvolk/rkt,joshix/rkt,kinvolk/rkt