docs(ngMock): fix example for $http respond() helper with function as 1st param

The  `whenPOST` method should return a response object containing status, response body and headers.
If omitted the following error will be thrown:

`Uncaught TypeError: Cannot read property '2' of undefined`

The documentation doesn't make it very clear, so I think it will be appropriate to add it here.

Closes #6761

src/ngMock/angular-mocks.js

@@ -1793,7 +1793,9 @@ angular.module('ngMockE2E', ['ng']).config(['$provide', function($provide) {
  *
  *     // adds a new phone to the phones array
  *     $httpBackend.whenPOST('/phones').respond(function(method, url, data) {
- *       phones.push(angular.fromJson(data));
+ *       var phone = angular.fromJson(data);
+ *       phones.push(phone);
+ *       return [200, phone, {}];
  *     });
  *     $httpBackend.whenGET(/^\/templates\//).passThrough();
  *     //...