INSTALL.md: proofread for grammar and formatting

It seems Wheezy is missing `pip3` (or at least an alias to `pip-3.2`). Python 3.2 is the only Python 3 version available in Wheezy.

Sid (unstable) and jessie (testing) are on Python 3.4.

The specifics for this document to cover are actually more complicated than I'd imagined; I didn't test any of the commands myself, I just did some proofreading and submitted the changes.