ModuleNotFoundError: No module named 'RPi' in calibrate_servos.py

A slack member reported this issue when running `python3 calibrate_servos.py 0 150`. The fixed that worked for them was to `pip install RPi.GPIO`. It's not clear whether that should be the case for everyone and thus added to instructions.

The way to replicate would be to start with a fresh install or use a virtualenv or similar.