Minor fix for PhutilParserGenerator

Summary: phutil_var_export only accepts a single parameter. This bug was introduced in D9925.

Test Plan: Eye-ball it.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9929