Update examples to use single objects instead of arrays

This replaces arrays with a single object as defined in https://github.com/json-api/json-api/pull/237

I'm not sure, but on the front page it might look better to leave the array and add a second post.. 
I'm not sure this instance should be changed. My understanding of the spec is that a single resource such as `/posts/1` could be represented as a single object, where as a collection of resources `/posts` should be represented as a collection, even if there is only one item in the collection.