docs(fix): Removed unused import (#1394)

Author: crisz

docs/auth/getting-started.md

@@ -10,7 +10,6 @@ the Firebase docs for more information on what methods are available.](https://f
 
 ```ts
 import { Component } from '@angular/core';
-import { Observable } from 'rxjs/Observable';
 import { AngularFireAuth } from 'angularfire2/auth';
 import * as firebase from 'firebase/app';