removed mention of for loop

Following up 2014-06-24-wise-penn/#2 It had been commented out here, and to minimise confusion I've deleted it completely. Loops come in lesson three (this is lesson one), so I think it's safe to exclude it from here.
removed mention of for loop

making the md file consistent with the Rmd file
Merge pull request #547 from benmarwick/remove-mention-of-loop-in-novice/r

removed mention of for loop in lesson one