Fix broken unit-test link.

* org-tests/index.org: Fix broken unit-test link

org-tests/index.org

@@ -141,7 +141,7 @@ you may install these extra utilities with the following git commands.
 * Ideas for test
 
 - Verify the output of tests with tools like =diff= (think /export/ here).
-- http://www.emacswiki.org/emacs-fr/UniitTesting
+- http://www.emacswiki.org/emacs-fr/UnitTesting
 
 * What we need to know to actually write tests