Fix function call: don't use non-existent argument.

Hi,

I'm trying to improve pyflakes (it's static code analyzer for Python https://github.com/pyflakes/pyflakes) so it will be able to detect function calls errors (like UnexpectedArgument).

As a part of testing I ran my version of pyflakes on your project (because it's quite popular Python project ^_^).
It found this bug.

Please review the change.
Thanks.