minor: fix formatting in contributing.rst and added links to virtualenv and virtualenvwrapper

Whoops, I'm used to markdown.  In rst, you need double backticks to make inline code blocks.  This commit fixes that formatting issue and adds a link to the docs of virtualenv and virtualenvwrapper.

These two changes could be separated, but I figured they're both formatting related and in the same file, so it made sense to put them in one pull request.  In the future, should I do separate pull requests?  Separate commits in the same pull request?  Or is this fine?