Update fixture adapter docs to work with ember-data.beta.3

I realize the fixture adapter docs are not linked to anywhere on the current website but I managed to stumble across them via a google search for "ember fixture data" when I was attempting to debug an unrelated issue. The example code in the current docs throws errors when used with the latest release of ember data `v1.0.0-beta.3`. This pr updates the examples to work with the latest version of ember data.


My replacement example for `App.Documenter.find(1)` is not as concise as the original example. If anyone has a better suggestion I'd love to hear it.