Clarified filenames for generating the API keys (kubernetes#12508)

content/en/docs/concepts/cluster-administration/certificates.md

@@ -183,7 +183,7 @@ Finally, add the same parameters into the API server start parameters.
 
         ../cfssl gencert -initca ca-csr.json | ../cfssljson -bare ca
 1.  Create a JSON config file for generating keys and certificates for the API
-    server as shown below. Be sure to replace the values in angle brackets with
+    server, for example, `server-csr.json`. Be sure to replace the values in angle brackets with
     real values you want to use. The `MASTER_CLUSTER_IP` is the service cluster
     IP for the API server as described in previous subsection.
     The sample below also assumes that you are using `cluster.local` as the default