Physical Computing - multiple minor documentation issues

[DeeJay]

1. Introduction [0] says "There are no additional software requirements for this resource beyond what is pre-installed in the current Raspbian image."

But Analogue Inputs [13] requires installation of additional packages.

2. The 'official' name of the recommended OS has been RasPiOS for some time.

3. Analogue Inputs [13] -
   a. The usual invocation of apt is now " sudo apt install "
   b. Tutorial fails to mention the need to "sudo apt update" prior to attempting package installation.

4. The tutorial recommends the use of the Mu editor. This seems to have become 'abandonware' in terms of support under RasPiOS.