Documentation/integrations.md: typo fix
Documentation/integrations.md: typo fix

Change "open an extensible" (a typo/incorrect) to "open and extensible".
kinvolk/rkt,ybubnov/rkt,rkt/rkt,lucasallan/rkt,rkt/rkt,coreos/rkt,joshix/rkt,Shashwatsh/rkt,ybubnov/rkt,ybubnov/rkt,Shashwatsh/rkt,kinvolk/rkt,lucasallan/rkt,lucasallan/rkt,alban/rkt,joshix/rkt,rkt/rkt,Shashwatsh/rkt,sstabellini/rkt,kinvolk/rkt,sstabellini/rkt,alban/rkt,lucasallan/rkt,intelsdi-x/rkt,sstabellini/rkt,joshix/rkt,kinvolk/rkt,rkt/rkt,coreos/rkt,intelsdi-x/rkt,intelsdi-x/rkt,coreos/rkt,alban/rkt,joshix/rkt