Simplify knitr example

[ethanwhite]

The knitr example currently involves a fairly complex dplyr pipeline and graph. We really just want to get across the basics so this example should be simplified, probably to a simply bi-variate scatter plot with a dataset that already has the data formated that way to allow the core information to be clearly demonstrated without the extra complexity.

The current version of the lecture example could be used as a simple exercise.

[ethanwhite]

Done by reducing the complexity of the existing example