Fix a typo in ManiphestTaskQuery

Summary: This doesn't have any impact on anything right now (we don't persist the query) but could in the future, so I just left it as-is but fixed the typo.

Test Plan: looked at it carefully

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: WikiChad, epriestley

Differential Revision: https://secure.phabricator.com/D9155