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