Fix broken link in phabricator documentation

Summary:
The link to libphutil Libraries User Guide was broken on Arcanist User
Guide: Customizing Lint, Unit Tests and Workflows.

Test Plan: ran diviner

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1890

src/docs/userguide/arcanist_lint_unit.diviner

@@ -33,7 +33,7 @@ make this work, you need to do three things:
 = Create a libphutil Library =
 
 If you haven't created a library for the class to live in yet, you need to do
-that first. Follow the instructions in @{article:libphutil Library User Guide},
+that first. Follow the instructions in @{article:libphutil Libraries User Guide},
 then make the library loadable by adding it to your ##.arcconfig## like this:
 
   {