Update the example to show json decoding

[simobiggs]

Simple example, but might save a bit of head scratching. Particularly querying an _id field like you get from faunadb was a bit confusing at first.

[kevmo314]

+1 to this PR, I spent a decent chunk of time trying to figure this out. It might be "obvious" because the code only unmarshals the json, but a simple example to make the example code actually successfully compile would go a long way.