Fix doc generation test
Fix doc generation test

* Correct the expected path for isfile
* Ensure isfile returns false.  Otherwise it will try
  to open the example rst file resulting in the test
  raising an OSError
* Replace nonexistent assert_has_call_with method with
  manual assertEquals on the last element of call_args_list

Verified this passes with mock 1.3.0.

boto/boto3