v2: Renamed --namespace flag to --namespaces

[vikkyomkar]

What this PR does / why we need it:
This PR renames --namespace flag to --namespaces
Which issue(s) this PR fixes *
Fixes #978

[olivierlemasle]

@vikkyomkar CI is failing because of an extra space in cli-arguments.md.
Actually, this file is auto-generated, you can fix it by running make generate.

To fix the issue, you can run:

make generate
git add docs/cli-arguments.md
git commit --amend
git push -f

Thanks!

[tariq1890]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: tariq1890, vikkyomkar

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