Fix xsprintf doclink to qsprintf

Summary: qsprintf() lives in Phabricator, not libphutil.

Test Plan: Generated, clicked docs.

Reviewers: btrahan, jungejason

Reviewed By: jungejason

CC: aran, epriestley

Maniphest Tasks: T938

Differential Revision: https://secure.phabricator.com/D1777