Datastore tutorial using wrong variable

On Cloud datastore documentation code:

``` javascript
// Save data to your dataset.
var blogPostData = {
  title: 'How to make the perfect homemade pasta',
  tags: ['pasta', 'homemade'],
  author: 'Andrew Chilton',
  isDraft: true,
  wordCount: 450
};

var blogPostKey = ds.key('BlogPost'); // ds.key(..) should be dataset.key(..)
```
