Update pre-requisites for R lessons

[maneesha]

In the Summary and Setup page the prerequisites for this lesson are:

Understand that computers store data and instructions (programs, scripts etc.) in files. Files are organised in directories (folders). Know how to access files not in the working directory by specifying the path.

This challenge exercise assumes some familiarity with a bash command line. Consider updating the prerequisites to note this, or update the challenge to note this requires concepts not covered in this lesson.