Fix example FBLogin in README.md

The `.auth` was missing in the example for the Facebook Login. It throws an error if you copy/paste the example and run it `firestack.signInWithProvider is not a function`.

Author: gizmodesbois

README.md

@@ -382,7 +382,7 @@ var Login = React.createClass({
  console.log("Logged in!");
  console.log(data);
  let token = data.credentials.token
- firestack.signInWithProvider('facebook', token, '') // facebook need only access token.
+ firestack.auth.signInWithProvider('facebook', token, '') // facebook need only access token.
  .then((user)=>{
  console.log(user)
  })