fix documentation typo
fix documentation typo

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