doc: trust: fix example
doc: trust: fix example

Fixes https://github.com/coreos/rkt/issues/1984

joshix/rkt,Wilybald/rkt,sstabellini/rkt,kinvolk/rkt,jakon89/rkt,jonboulle/rkt,mike1729/rkt,endocode/rkt,coreos/rkt,achanda/rkt,kinvolk/rkt,cab105/rkt,shobhit85/rkt,Shashwatsh/rkt,matthaias/rkt,intelsdi-x/rkt,joshix/rkt,dgonyeo/rkt,sstabellini/rkt,jjlakis/rkt,akshaykarle/rkt,jonboulle/rkt,nhlfr/rkt,joshix/rkt,lucasallan/rkt,mike1729/rkt,ybubnov/rkt,akshaykarle/rkt,cab105/rkt,blixtra/rkt,rkt/rkt,monstermunchkin/rkt,sgotti/rkt,lucasallan/rkt,intelsdi-x/rkt,linki/rkt,yifan-gu/rkt,squeed/rkt,kinvolk/rkt,blixtra/rkt,joshix/rkt,lucasallan/rkt,yifan-gu/rocket,kinvolk/rkt,endocode/rkt,s-urbaniak/rkt,lucasallan/rkt,tmrts/rkt,coreos/rkt,alban/rkt,il9ue/rkt,sgotti/rkt,Wilybald/rkt,nhlfr/rkt,rkt/rkt,sgotti/rkt,Wilybald/rkt,matthaias/rkt,alban/rkt,monstermunchkin/rkt,linki/rkt,rkt/rkt,blixtra/rkt,yifan-gu/rocket,sinfomicien/rkt,euank/rkt,shobhit85/rkt,sinfomicien/rkt,s-urbaniak/rkt,cab105/rkt,achanda/rkt,dgonyeo/rkt,akshaykarle/rkt,yifan-gu/rkt,Shashwatsh/rkt,mike1729/rkt,markllama/rkt,ybubnov/rkt,sstabellini/rkt,squeed/rkt,sinfomicien/rkt,ybubnov/rkt,jjlakis/rkt,jakon89/rkt,squeed/rkt,markllama/rkt,shobhit85/rkt,nhlfr/rkt,jonboulle/rkt,endocode/rkt,il9ue/rkt,Wilybald/rkt,intelsdi-x/rkt,jakon89/rkt,coreos/rkt,rkt/rkt,yifan-gu/rkt,jjlakis/rkt,euank/rkt,euank/rkt,markllama/rkt,alban/rkt,kbrwn/rkt,monstermunchkin/rkt,s-urbaniak/rkt,Shashwatsh/rkt,yifan-gu/rocket,matthaias/rkt,dgonyeo/rkt,tmrts/rkt,kbrwn/rkt,kbrwn/rkt,linki/rkt,tmrts/rkt,achanda/rkt,il9ue/rkt