KEP-3257: fix ClusterRole example

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

Author: dgn

keps/sig-auth/3257-cluster-trust-bundles/README.md

@@ -447,7 +447,7 @@ rules:
 - apiGroups:
   - certificates.k8s.io
   resources:
-  - signer
+  - signers
   resourceNames:
   - example.com/my-signer
   verbs: