Fix typo.
Invoke git with no arguments to see if it is installed.

Encouraging the use of `which git` is confusing, because the failure state
is an empty response. Since the bare `git` command yields help text, and
failure response is straight-forward ("-bash: git: command not found\n"),
we should encourage a just-try-it approach.
Git != GitHub

While most of us use GitHub as a primary remote, It shouldn't be taught as
the *reason* git is awesome. There are plenty of workflows that are possible
only outside of the github ecosystem, and the features we're selling here
-- specifically the distributed part of DVCS -- are something that git does
well on its own, so it makes sense to mention them as features and *then*
introduce GitHub as a service that adds value to these features.
Merge pull request #5 from yaauie/git-lesson-improvements

Git lesson improvements