Fixing demo source reference

Author: mzabriskie

example/index.html

@@ -74,7 +74,7 @@
         <div>
           <h1>React Draggable</h1>
           <p>
-            <a href="https://github.com/mzabriskie/react-draggable/blob/master/example/main.js">Demo Source</a>
+            <a href="https://github.com/mzabriskie/react-draggable/blob/master/example/index.html">Demo Source</a>
           </p>
           <Draggable zIndex={100}>
             <div className="box">I can be dragged anywhere</div>