Update typo in code sample in reference.md (#795)

docs/reference.md

@@ -1105,7 +1105,7 @@ And create a new instance:
 ```js
 // create a User object from our Factory
 app.factory("User", function(UserFactory) {
-  var ref = firebase.database.ref();
+  var ref = firebase.database().ref();
   var usersRef = ref.child("users");
   return function(userid) {
     return new UserFactory(ref.child(userid));