Fixing a typo in libphutil

Summary: dipsose raises errors when you try to update your install

Test Plan:
Ran a update script, it failed when it tried to generate diviner documentation
fixed the typo and ran it again, it succeeded

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1842