[Doc] Minor fix to MKP secret setup guide

[numerology]

Change the way of specifying namespace.

Sometimes --current option might not work for gcloud in MacOS, causing empty secret and permission issues.

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This change is 

[numerology]

/assign @rmgogogo
/assign @Bobgy

[Bobgy]

/lgtm
I totally agree with this.

left a minor concern

[rmgogogo]

Thanks, give hours to do more test on it.
I find something wrong in my existing cluster, wondering whether we have other issue. (not sure whether you tested the new CLI)

[numerology]

Thanks, give hours to do more test on it.
I find something wrong in my existing cluster, wondering whether we have other issue. (not sure whether you tested the new CLI)

What's the symptom? I did not try out the new cli.

[rmgogogo]

tested and all works well.

/lgtm

[Bobgy]

@IronPan for approval
@rmgogogo Can you approve?

[IronPan]

/approve

[IronPan]

/lgtm

[k8s-ci-robot]

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[numerology]

k8s robot seems stop working, manually merged