Cite shell parameter expansion

This is part of Issue #54.

In this commit we mention the shell parameter expansion to make
easy to students go deep if they need.

We also add a note about use ${some-var} instead of $some-var.
Fix some typos

Thanks to @ahmadia and @ethanwhite for the review.
Merge pull request #59 from r-gaia-cs/teach06-expansion

Cite shell parameter expansion