Correct Arcanist repository hooks documentation
Summary: Long ago this was a symlink, but now it's a shell script because symlinks and Windows don't really work. Fixes T2884.
Test Plan: Inspection.
Reviewers: btrahan
Reviewed By: btrahan
CC: aran
Maniphest Tasks: T2884
Differential Revision: https://secure.phabricator.com/D5575