Propose re-ordering hypothesis testing section in episode 2

[mallewellyn]

Propose:

• Moving the p-values challenge (2) to after the full description of p values etc to test how well learners have re-capped/understood this content
• Propose putting the stuff about fitting a lot of linear models at the end of the hypothesis testing section and combining the t stats stuff into the main hypothesis testing for flow
• Some minor rewording just to clarify the above

[mallewellyn]

Added a couple of other changes to episodes 6 and 7 above to align challenges with objectives defined at the start of the episode

[mallewellyn]

Use prostate instead of cars in above commit