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