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