Added the key attribute into the todo example

This is to solve the warning (that the key should be set) that was being raised when adding a new item.

docs/_js/examples/todo.js

@@ -1,8 +1,8 @@
 var TODO_COMPONENT = `
 var TodoList = React.createClass({
   render: function() {
-    var createItem = function(itemText) {
-      return <li>{itemText}</li>;
+    var createItem = function(itemText, index) {
+      return <li key={index}>{itemText}</li>;
     };
     return <ul>{this.props.items.map(createItem)}</ul>;
   }