create-usb: Tweak docs for --destination-repo
create-usb: Tweak docs for --destination-repo

Make it show up in the help output as --destination-repo=DEST so it's
clear that it takes an argument.

Closes: #1656
Approved by: jlebon

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree