grpc example has an incorrect version for knative and incorrect tag to push

[houshengbo]

Expected Behavior

Successful execution of this example.

Actual Behavior

Docker push command uses a wrong tag name, and knative should pin to 0.6 or later.

Steps to Reproduce the Problem

1. Follow the instruction at https://github.com/knative/docs/tree/master/docs/serving/samples/grpc-ping-go

Additional Info

Additional context
Add any other context about the problem here.

Install information:

• Platform (GKE, IKS, AKS, etc.):
• Knative Version: