Fix syntax error in authListener

use firestack instead of firebase in auth listener code example

Author: florianbepunkt

docs/api/authentication.md

@@ -26,7 +26,7 @@ class Example extends React.Component {
   }
 
   componentDidMount() {
-    this.unsubscribe = firebase.auth().onAuthStateChanged((user) => {
+    this.unsubscribe = firestack.auth().onAuthStateChanged((user) => {
       if (user) {
         // User is signed in.
       }