Fix Grunt link in docs

The repo has moved from 'cowboy/grunt' to 'gruntjs/grunt'.

Author: necolas

doc/TOC.md

@@ -31,6 +31,6 @@ of your site/app in various production environments.
 * [Server configs](https://github.com/h5bp/server-configs) — Configs for
  non-Apache servers.
 * [Node build script](https://github.com/h5bp/node-build-script) — A
- feature-rich [grunt](https://github.com/cowboy/grunt) plugin.
+ feature-rich [grunt](https://github.com/gruntjs/grunt) plugin.
 * [Ant build script](https://github.com/h5bp/ant-build-script) — The original
  HTML5 Boilerplate build script.