Fixed the link to the help page.

GitHub was interpreting the link as a local link because it didn’t have
the HTTP scheme name.
The StoryPage can now be updated in the datastore.

* Added an `update` method for updating an already existing model in
the datastore. This method was written in the `Model` with no
modifications to the `StoryPage` subclass, so any `Model` subclass that
implements the methods necessary for the `create` and `read` methods
will already have the methods necessary for the `update` method.
* Created a unit test to test the `update` method.