Fix 2 grammatical errors in README.md

This fixes a couple common grammatical errors in README.md, namely:

it's -> its

although normally putting an 's after a noun makes it possessive, in the case of "it", putting an s after without an ' makes it possessive.  "It's" is a contraction for "It is".

Here's the Oatmeal comic I learned all my knowledge from:

![totes awesome Oatmeal comic](http://images4.fanpop.com/image/photos/24100000/10-Words-You-Need-to-Stop-Misspelling-It-s-and-Its-english-language-24124229-676-570.png)