arduino_mega_pins.h: fix incorrect pin definition
arduino_mega_pins.h: fix incorrect pin definition

ambrop72/aprinter,fixator/aprinter,johnkingsley/aprinter,ambrop72/aprinter,fixator/aprinter,johnkingsley/aprinter,ambrop72/aprinter,fixator/aprinter,johnkingsley/aprinter,ambrop72/aprinter,ambrop72/aprinter,fixator/aprinter,fixator/aprinter,johnkingsley/aprinter,ambrop72/aprinter,fixator/aprinter,johnkingsley/aprinter,ambrop72/aprinter,johnkingsley/aprinter,johnkingsley/aprinter