drivers/ds18: fix doxygen group of ds18_internal.h
drivers/ds18: fix doxygen group of ds18_internal.h

authmillenon/RIOT,kaspar030/RIOT,yogo1212/RIOT,kaspar030/RIOT,OTAkeys/RIOT,authmillenon/RIOT,authmillenon/RIOT,kYc0o/RIOT,jasonatran/RIOT,kYc0o/RIOT,OTAkeys/RIOT,basilfx/RIOT,kaspar030/RIOT,yogo1212/RIOT,smlng/RIOT,jasonatran/RIOT,OTAkeys/RIOT,jasonatran/RIOT,miri64/RIOT,miri64/RIOT,ant9000/RIOT,OlegHahm/RIOT,kYc0o/RIOT,OTAkeys/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,miri64/RIOT,jasonatran/RIOT,yogo1212/RIOT,basilfx/RIOT,OlegHahm/RIOT,kaspar030/RIOT,miri64/RIOT,ant9000/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,smlng/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,smlng/RIOT,smlng/RIOT,kYc0o/RIOT,basilfx/RIOT,OlegHahm/RIOT,ant9000/RIOT,jasonatran/RIOT,OTAkeys/RIOT,ant9000/RIOT,OlegHahm/RIOT,yogo1212/RIOT,basilfx/RIOT,OlegHahm/RIOT,miri64/RIOT,authmillenon/RIOT,authmillenon/RIOT,yogo1212/RIOT,ant9000/RIOT,basilfx/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,yogo1212/RIOT,smlng/RIOT