Fix typo in extract function

        if os.path.exists(directory):

            if not os.path.isdir(directory):

                raise Exception("Destination %s is not a directory" % directory)

            if not force and len(os.listdir(dest)) > 0:

            if not force and len(os.listdir(directory)) > 0:

                raise Exception("Directory %s is not empty (need force)" % directory)

        else:

            istools.mkdir(directory)