k8s quickstart: Fix namespace in workload registration apply

[jsnctl]

Description of the change

Amends the namespace used when registering the workload in the Kubernetes Quickstart example. This is erroneously set to default in the example kubectl apply, it should be set to spire

Which issue this PR fixes

Not in this repo, but this was raised in spire-tutorials last September: https://github.com/spiffe/spire-tutorials/issues/123

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[akaloti]

Doesn't the tutorial command for the workload container put it in default namespace? (the kubectl apply -f client-deployment.yaml) If changing the namespace referenced in the registration entry, I think you'd need to change the command too.