doc: rename diagexec -> appexec
doc: rename diagexec -> appexec

Stage1's diagexec was renamed to appexec in https://github.com/coreos/rkt/pull/1627

Update the documentation accordingly.

[skip ci]

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