Docs: Fix thoughtbot link in testing guide

doc/development/testing_guide/index.md

@@ -4,7 +4,7 @@ This document describes various guidelines and best practices for automated
 testing of the GitLab project.
 
 It is meant to be an _extension_ of the [thoughtbot testing
-style guide](https://github.com/thoughtbot/guides/tree/master/style/testing). If
+style guide](https://github.com/thoughtbot/guides/tree/master/testing-rspec). If
 this guide defines a rule that contradicts the thoughtbot guide, this guide
 takes precedence. Some guidelines may be repeated verbatim to stress their
 importance.