Update GCP PubSub Page

[samodell]

Expected Behavior

https://knative.dev/docs/eventing/samples/gcp-pubsub-source/ is easy to follow

Actual Behavior

Install instructions for Knative are unclear. It's also not clear when you are done with the install portion.

Additional context

Screenshots from Slack discussion w/ Jonathan Campos: https://drive.google.com/file/d/1ceAyeFiQRyYougY5PTy8NrOI6B63nNQ0/view

https://drive.google.com/file/d/149DtUDgx9din0Ys2fS9aD-grjaMiqoFe/view

(Must be a member of knative-users to view: https://groups.google.com/forum/#!forum/knative-users )

[nachocano]

/assign