Subsetting.Rmd: proposes a few small things for consistency

* small changes to comments to make boolean/integer comparisons consistent
* last section changed to "Exercises" & bullets changed to numbers to match Data-structures.Rmd

These are more matters of style, rather than typos

Other questions: 

* I understand that subset() is to be used with some care within programs (for reasons I'm sure I will understand better as I make my way through the rest of the material). Is it worth repeating these words of caution, or should we know this already if we have got this far in R?

* I have read (but can't remember the references) about your aversion to using rownames(), esp. in plyr. I imagine what you showed in this chapter is kosher because you are not letting the row names out of your supervision, as they are used in the next line of code. Is this the place for words of caution about using row names more generally?