Fix small typo in comments of system.c
Fix small typo in comments of system.c

zegervdv/Quadcopter,zegervdv/Quadcopter,zegervdv/STM-Quadcopter,zegervdv/Quadcopter,zegervdv/STM-Quadcopter,zegervdv/STM-Quadcopter,zegervdv/Quadcopter,zegervdv/STM-Quadcopter