update firebase auth code example

Example 1: firebase auth update

In the newest version of firebase_auth:

FirebaseUser has been changed to User

AuthResult has been changed to UserCredential

GoogleAuthProvider.getCredential() has been changed to GoogleAuthProvider.credential()

onAuthStateChanged which notifies about changes to the user's sign-in state was replaced with authStateChanges()

currentUser() which is a method to retrieve the currently logged in user, was replaced with the property currentUser and it no longer returns a Future<FirebaseUser>

Example 2: firebase auth update current user

var user = firebase.auth().currentUser;user.updateProfile({  displayName: "Jane Q. User",  photoURL: "https://example.com/jane-q-user/profile.jpg"}).then(function() {  // Update successful.}).catch(function(error) {  // An error happened.});