Fix animation example code

key should never be index into an array or there are bugs. Especially in
transitions.

Fixes facebook#853

Author: zpao

docs/docs/09.1-animation.md

@@ -35,7 +35,7 @@ var TodoList = React.createClass({
   render: function() {
     var items = this.state.items.map(function(item, i) {
       return (
-        <div key={i} onClick={this.handleRemove.bind(this, i)}>
+        <div key={item} onClick={this.handleRemove.bind(this, i)}>
           {item}
         </div>
       );