Update R_pandas_compare.md

Hi April, That's nice and it'll be very useful!

  I can only comment on the R part, I'm suggesting some edits for consistency of spaces and a few other things.

  Another comment is that `df` is actually an R function and so it would be best to call the data `dat` or something else.

  Thanks!
Merge pull request #7 from fmichonneau/patch-1

Good catch, Francois. I'll make the edits to the data frame name.