docs($resource): fix example using promise

Author: wesleycho

src/ngResource/resource.js

@@ -253,7 +253,7 @@ function shallowClearAndCopy(src, dst) {
 
    ```js
      var User = $resource('/user/:userId', {userId:'@id'});
-     var user = User.get({userId:123}, function() {
+     User.get({userId:123}, function(user) {
        user.abc = true;
        user.$save();
      });
@@ -273,6 +273,16 @@ function shallowClearAndCopy(src, dst) {
        });
      });
    ```
+ *
+ * You can also access the raw `$http` promise via the `$promise` property on the object returned
+ *
+   ```
+     var User = $resource('/user/:userId', {userId:'@id'});
+     User.get({userId:123})
+         .$promise.then(function(user) {
+           $scope.user = user;
+         });
+   ```
 
  * # Creating a custom 'PUT' request
  * In this example we create a custom method on our resource to make a PUT request