demo/choose_clock: Fix a printf.
demo/choose_clock: Fix a printf.

"Selecting master clock %d for channel %d on subdevice %d.\n"

The %d after subdevice was missing although the corresponding parameter
in the argument list was present.

olsonse/comedilib,Linux-Comedi/comedilib,olsonse/comedilib,Linux-Comedi/comedilib,Linux-Comedi/comedilib,Linux-Comedi/comedilib,olsonse/comedilib,olsonse/comedilib,olsonse/comedilib,Linux-Comedi/comedilib,olsonse/comedilib,Linux-Comedi/comedilib