Fix broken internal link

docs/testing/setup.md

 The easiest way to get setup it to use buildkit. Buildkit will install the tools
 necessary for running the tests and creating local CiviCRM sites.
 
-You can download [CiviCRM buildkit][buildkit] which includes all the test tools. 
+You can download [CiviCRM buildkit](/tools/buildkit) which includes all the test tools.
 From there you can create a test site using [these instructions][buildkit-create].
 
 !!! info 
     When writing new tests or making any changes make sure you run [civilint] 
     to ensure your changes match our coding style.
 
-[buildkit]: tools/buildkit.md
 [buildkit-create]: https://buildkit.civicrm.org/#/tutorials