Update object model spec with v1 api

[aliok]

Describe the bug
https://github.com/knative/eventing/blob/master/docs/spec/spec.md talks about v1beta1 API.

Expected behavior
https://github.com/knative/eventing/blob/master/docs/spec/spec.md talks about v1 API :)

To Reproduce
Steps to reproduce the behavior.

Knative release version

Additional context
Add any other context about the problem here such as proposed priority

[lberk]

Do we know how these documents are used? (This is the first I'm hearing about them -- just want to make sure we aren't manually generating this or generating bad docs from an out of date source)

[aliok]

/unassign aliok

The task here is to first find out if these docs are still used, how they are created, ...

[AlexandraRoatis]

/assign