Update Subsetting.rmd

fixed typo (subsetting changed to simplifying)
Update Subsetting.rmd

Why not make the example consistent with the text?
Update Subsetting.rmd
disregard this edit

Hadley, this doesn't format correctly in the PDF (at least in the version you sent me last Thursday (4/24/2014). I put my comment into the source code just to make it easy to identify exactly where the problem occurs.
Merge pull request #473 from twjacobs/master

Update Subsetting.rmd