Update README.md

Closes #77

Author: posva

README.md

@@ -70,15 +70,15 @@ var vm = new Vue({
 })
 ```
 
-**About the cancelCallback**: This callback is actually an error handler that
-is called if the read/write doesn't succeed. You can of course also use it with
-`asObject: false` and apply it to arrays. The firebase doc states:
+**About the cancelCallback**:
 
 > We can pass an optional cancel callback to our event subscription that will
 > be called if the read is ever cancelled. A read would be cancelled if the app
 > client doesn't have permission to read from that database reference. This
 > callback will be passed an error object indicating why the failure occurred.
 
+[From Firebase docs](https://firebase.google.com/docs/database/admin/retrieve-data)
+
 ``` html
 <div id="demo">
   <pre>{{ anObject }}</pre>