Fix firmware build documentation
Fix firmware build documentation

CMake doesn't automatically create a target by the same name of the
project, so `greatfet_usb` is not a target generated by CMake, and it
probably makes more sense to just build all targets which includes
`greatfet_usb.bin`.

dominicgs/GreatFET-experimental,greatscottgadgets/greatfet,greatscottgadgets/greatfet,greatscottgadgets/greatfet,greatscottgadgets/greatfet,dominicgs/GreatFET-experimental,dominicgs/GreatFET-experimental