Update Kubernetes deployment docs example to avoid confusion

[tsandall]

The Deployment docs for Kubernetes include an example query that provides a Pod object as input. This is confusing for new users because they expect to be able to write admission control policies that refer to resources rooted at input -- whereas Kubernetes admission control webhooks root the resource under input.request.object.

We should just replace the example in the Deployment docs with something generic and then refer to readers to the Admission Control tutorial if they are interested.

The doc in question: https://github.com/open-policy-agent/opa/blob/master/docs/book/deployments.md#kicking-the-tires