Update README.md

Changing findUserByEmail(), findUserByUserID() and findUserById() to getUser().

After trying to work out why findUserByUserID() did not work, I realised it should just be getUser().
The example in the Docs - http://doc.intercom.io/api/#view-a-user show this correctly, but the README does not.
Merge pull request #56 from papertrailrob/patch-1

Update README.md