Improve running test instructions and denote commands with $

[colby-swandale]

This PR is adjusting a couple of things:

Improve the instructions around running Rake's test suite. I think the instructions may have been left-over from a previous form and wasn't making a huge amount of sense.

The second is denoting commands with the $ prefix. This is a typical convention in documentation and helpful for new contributors.

[coveralls]

Coverage increased (+0.1%) to 97.56% when pulling c664dde on colby-swandale:colby/improve-contributing-doc into 628633c on ruby:master.