Make indentation consistent.
Fix typos.
Change instructions to edit `.hg/hgrc` instead of using `hg config`.

`hg config` is an extension that doesn't ship with Mercurial
(see http://mercurial.selenic.com/wiki/ConfigExtension)
and the story of extensions, especially external ones, is out of scope
for a novice lesson.
Fix arrow in image for pull to local repo.

re: https://github.com/swcarpentry/bc/pull/439#discussion_r13412113
Adjust page sizes to eliminate excessive whitespace below images.
Fix direction of arrow & adjust page size to eliminate excess whitespace.
Delete left over git image files.