Small edits/additions to the novice shell lessons

I taught a bootcamp yesterday using the new novice shell lessons. In preparing for that bootcamp I found a few bugs in the challenges and a few missing data files, so I've addressed these in this PR.

(for some reason the PR is comparing the `swcarpentry:gh-pages` branch to my new `DamienIrving:shell-edits` branch, however I had envisaged merging my branch with `swcarpentry:master`, which is where the new shell lessons are...)