docs: update demo source code url

Author: UziTech

README.md

@@ -7,7 +7,7 @@ It works in all modern desktop and mobile browsers and doesn't depend on any ext
 
 ## Demo
 
-[Demo](http://szimek.github.io/signature_pad) works in desktop and mobile browsers. You can check out its [source code](https://github.com/szimek/signature_pad/blob/gh-pages/js/app.js) for some tips on how to handle window resize and high DPI screens. You can also find more about the latter in [HTML5 Rocks tutorial](http://www.html5rocks.com/en/tutorials/canvas/hidpi).
+[Demo](http://szimek.github.io/signature_pad) works in desktop and mobile browsers. You can check out its [source code](https://github.com/szimek/signature_pad/blob/master/docs/js/app.js) for some tips on how to handle window resize and high DPI screens. You can also find more about the latter in [HTML5 Rocks tutorial](http://www.html5rocks.com/en/tutorials/canvas/hidpi).
 
 ### Other demos