Fix an ExecFuture typo, "preprebuilt"
Fix an ExecFuture typo, "preprebuilt"

Summary: These commands are merely "prebuilt", not "preprebuilt".

Test Plan: o.O

Differential Revision: https://secure.phabricator.com/D21702

codelabs-ch/arcanist,phacility/arcanist,codelabs-ch/arcanist,phacility/arcanist,phacility/arcanist,phacility/arcanist,codelabs-ch/arcanist,codelabs-ch/arcanist,codelabs-ch/arcanist,phacility/arcanist