Add short section on vector recycling

This adds a short section on how R recycles vectors when dealing with vectors of unequal length. In order to render the .md file I also had to change the pattern in the `list.files` call on line 32, as there are more than just the original inflammation csv's that match the "csv" pattern.

This PR is the "small fix" submitted as part of the final assignment for the Software Carpentry instructor training.