fix typo in README.md

This change alters two spelling/grammar mistakes.  ("it's" should be "its", "an" should b "a".)  No other changes have been made.

Before: Each API call takes a callback function as it's first argument.  Upon an result this function is called...
After: Each API call takes a callback function as its first argument.  Upon a result this function is called...
Merge pull request #3 from lpappone/patch-1

fix typo in README.md