Correction needed to Arduino code for activity 2.5.7

It should be an analogWrite() and not a digitalWrite() to activate a PWM output. Code needs correction in the PDF and on the GitHub repository.

It might be nice to have a separate section for documentation corrections and maybe a procedure for submitting pull requests. Now it is not clear how to best report documentation problems. The formatting of the Arduino code leaves a lot to be desired. I would prefer a change to a monospaced font to differentiate it more from the main text.

BTW I received my picobricks yesterday in western US. It was a relief to have the package released from UPS hold. I am enjoying the setup. It is nice and clean that the main board works out of the box without having to setup any cabling. This is by far the easiest way I have been introduced to a microcontroller. After having used MicroPython and JavaScript for a couple of projects, I had completely forgotten what a pain Arduino C++. (It is really C++ and not C, because the libraries are imported as objects and use the dot notation.)