fix phpdoc comment

Fixed small inaccuracy: in `\GuzzleHttp\Adapter\TransactionIterator` options called `['before', 'complete', 'error']` (http://guzzle.readthedocs.org/en/latest/clients.html#asynchronous-response-handling).

See https://github.com/guzzle/guzzle/blob/4.2.2/src/Adapter/TransactionIterator.php#L34