Update the parallel,sequence to flows.knative.dev

[vaikas]

What is changing? (Please include as many details as possible.)

We're moving parallel & sequence from messaging.knative.dev to flows.knative.dev API group. We need to update our docs to reflect this.

How will this impact our users?

They should move to the new flows.knative.dev resources.

In what release will this take happen (to the best of your knowledge)?

v0.11

Context

Link to associated PRs or issues from other repos here.

1. Sequence and Parallel in different API group ?  eventing#1913

Additional info