Update 02-Shell-variables.md
- Corrected some typos and language errors.
- Added a
$
to indicate console prompt - so that it is easier to see what is input and what is output - Tried to add some more explanation and/or context to the examples
- Moved things around to avoid repetition and hopefully keep a clearer story path
- Replaced the === to ```{note} directive
Edited by Maiken Pedersen