Fix broken link in readme

[yankay]

What type of PR is this?
/kind bug

What this PR does / why we need it:
From the #8827, the centos.md has been renamed. So the link in readme is broken.
The PR is to fix the broken link in readme

Which issue(s) this PR fixes:
None

Special notes for your reviewer:
None

[k8s-ci-robot]

Hi @yankay. 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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[floryut]

/ok-to-test

@yankay Thank you, always a pain to keep md up to date

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: floryut, yankay

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[oomichi]

/lgtm

[k8s-ci-robot]

New changes are detected. LGTM label has been removed.