Fix the link to the doc

gui/readme.md

@@ -2,4 +2,5 @@
 The Babylon.js GUI library is an extension you can use to generate interactive user interface.
 It is build on top of the DynamicTexture.
 
-Documentation: http://doc.babylonjs.com/overviews/gui
+Documentation: http://doc.babylonjs.com/how_to/gui
+API: http://doc.babylonjs.com/api/modules/babylon.gui