📖 Fix typos

[tsuzu]

What this PR does / why we need it:
This PR fixes some typos in documents.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):

/area documentation

[k8s-ci-robot]

Hi @tsuzu. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[sbueringer]

Thx
/ok-to-test
/lgtm
/approve

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: 3975010416769d16a2c3f9429161e0163a09cc65

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: sbueringer

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