fix grammar in license header

boards/native/board_config.c

boards/native/board_init.c

boards/native/drivers/native-led.c

boards/native/drivers/native-ltc4150.c

boards/native/include/board.h

boards/telosb/board.c

boards/telosb/uart.c

core/bitarithm.c

core/clist.c

core/hwtimer.c

core/include/atomic.h

core/include/bitarithm.h

core/include/clist.h

core/include/debug.h

core/include/flags.h

core/include/hwtimer.h

core/include/hwtimer_arch.h

core/include/kernel.h

core/kernel_init.c

core/msg.c