KEP-3257: fix ClusterRole example

[dgn]

• One-line PR description:
  Corrects the example ClusterRole in the README.md of KEP-3257

• Issue link:
  ClusterTrustBundles (previously Trust Anchor Sets) #3257

• Other comments:
  It was using singular "signer" which doesn't work. It has to be "signers", like in the tests: https://github.com/kubernetes/kubernetes/blob/6b8e5a9457a015e5568d7ae21b94d8a4eeb41bb3/test/integration/clustertrustbundles/admission_establishtrust_test.go#L142

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[enj]

/lgtm
/approve
/ok-to-test

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dgn, enj

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