Fix XHPAST build instructions

Summary: Fixes T7242. This is a typo from D11517.

Test Plan: N/A

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7242

Differential Revision: https://secure.phabricator.com/D11758