Fix minor grammatical problem

"check out" makes more sense here.

README.md

@@ -6,7 +6,7 @@
 
 Bootstrap is a sleek, intuitive, and powerful front-end framework for faster and easier web development, created and maintained by [Mark Otto](http://twitter.com/mdo) and [Jacob Thornton](http://twitter.com/fat).
 
-To get started, checkout [http://getbootstrap.com](http://getbootstrap.com)!
+To get started, check out [http://getbootstrap.com](http://getbootstrap.com)!