[R] Fix typo in R/README.md
[R] Fix typo in R/README.md

Author: Sean Kross <smk240@gmail.com>

Closes #56 from seankross/master and squashes the following commits:

c6e7473 [Sean Kross] Update README.md

chris-b1/feather,chris-b1/feather,wesm/feather,chris-b1/feather,chris-b1/feather,wesm/feather,wesm/feather,wesm/feather,wesm/feather,chris-b1/feather,wesm/feather,wesm/feather,chris-b1/feather,wesm/feather