Fix incorrect use of "it's" in README

First off, thanks for the great documentation!

I made some minor changes to fix grammar in the README. "It's" is a contraction for "it is", not the possessive form of "it". There's a nice summary of the info @ http://www.its-not-its.info/

I signed the Chef ICLA. See https://supermarket.chef.io/users/jalessio