child() method missing

The call to child("messages") method is missing in line 145 thus, the Controller (ChatCtrl) won't be able to get the messages collection

docs/guide/synchronized-arrays.md

@@ -142,7 +142,7 @@ var app = angular.module("sampleApp", ["firebase"]);
 app.factory("chatMessages", ["$firebaseArray",
   function($firebaseArray) {
     // create a reference to the database where we will store our data
-    var ref = firebase.database().ref();
+    var ref = firebase.database().ref().child("messages");
 
     return $firebaseArray(ref);
   }