docs: update RPi_microcontroller.md to systemd

Signed-off-by: Stefan Dej <meteyou@gmail.com>

docs/RPi_microcontroller.md

@@ -25,8 +25,8 @@ must run before the klippy process.
 After installing Klipper, install the script. run:
 ```
 cd ~/klipper/
-sudo cp "./scripts/klipper-mcu-start.sh" /etc/init.d/klipper_mcu
-sudo update-rc.d klipper_mcu defaults
+sudo cp ./scripts/klipper-mcu.service /etc/systemd/system/
+sudo systemctl enable klipper-mcu.service
 ```
 
 ## Building the micro-controller code