boards/firefly: Fix units in doc
boards/firefly: Fix units in doc

miri64/RIOT,miri64/RIOT,basilfx/RIOT,jasonatran/RIOT,authmillenon/RIOT,ant9000/RIOT,basilfx/RIOT,kaspar030/RIOT,OlegHahm/RIOT,kYc0o/RIOT,basilfx/RIOT,miri64/RIOT,OlegHahm/RIOT,ant9000/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,kaspar030/RIOT,authmillenon/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,ant9000/RIOT,ant9000/RIOT,OlegHahm/RIOT,kYc0o/RIOT,jasonatran/RIOT,kYc0o/RIOT,jasonatran/RIOT,OTAkeys/RIOT,authmillenon/RIOT,kaspar030/RIOT,OTAkeys/RIOT,basilfx/RIOT,kaspar030/RIOT,kYc0o/RIOT,OTAkeys/RIOT,RIOT-OS/RIOT,miri64/RIOT,jasonatran/RIOT,ant9000/RIOT,RIOT-OS/RIOT,miri64/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,authmillenon/RIOT,kaspar030/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,basilfx/RIOT