fix validation message: apiServerEndpoints -> apiServerEndpoint

cmd/kubeadm/app/apis/kubeadm/validation/validation.go

@@ -148,7 +148,7 @@ func ValidateDiscoveryBootstrapToken(b *kubeadm.BootstrapTokenDiscovery, fldPath
 	}
 
 	allErrs = append(allErrs, ValidateToken(b.Token, fldPath.Child(kubeadmcmdoptions.TokenStr))...)
-	allErrs = append(allErrs, ValidateDiscoveryTokenAPIServer(b.APIServerEndpoint, fldPath.Child("apiServerEndpoints"))...)
+	allErrs = append(allErrs, ValidateDiscoveryTokenAPIServer(b.APIServerEndpoint, fldPath.Child("apiServerEndpoint"))...)
 
 	return allErrs
 }