out of date GKE example

[jmhodges]

The godoc GKE example are out of date. The "gke_container" type has changed to "k8s_container". I'm also not sure which of the given options are required.

(And while some of the downward API docs explains some of those env vars, it's not immediately clear that the user would have to fill in the env var for the container name in the deployment themselves, etc. But maybe that's mooted? I understand there's a scope problem here, but there's def some things that can be lost since neither the example nor the downward API docs are very explicit in how to connect them.)