Update section 1

[jf]

This is an update for lesson 5 containing the following changes:

• update clj invocation to include -M / remove deprecation warning
• refine explanations for clj invocation (I struggled for a bit with this one, trying to decide how much detail to include. In the end, I think I included sufficient detail to provide a good-enough understanding.. while I think still trying to keep to the spirit of the original explanation)

Let me know your thoughts?

It would also be nice to update the section on clj-new to (following clj-new's own README) install it as a tool; but that's a bigger update than I'm prepared to work on right now (and maybe this is better left to @kendru as the original author?)