ctlv3: have "protobuf" in output help string instead of "proto"

Fixes #7538

etcdctl/ctlv3/ctl.go

@@ -45,7 +45,7 @@ var (
 func init() {
 	rootCmd.PersistentFlags().StringSliceVar(&globalFlags.Endpoints, "endpoints", []string{"127.0.0.1:2379"}, "gRPC endpoints")
 
-	rootCmd.PersistentFlags().StringVarP(&globalFlags.OutputFormat, "write-out", "w", "simple", "set the output format (fields, json, proto, simple, table)")
+	rootCmd.PersistentFlags().StringVarP(&globalFlags.OutputFormat, "write-out", "w", "simple", "set the output format (fields, json, protobuf, simple, table)")
 	rootCmd.PersistentFlags().BoolVar(&globalFlags.IsHex, "hex", false, "print byte strings as hex encoded strings")
 
 	rootCmd.PersistentFlags().DurationVar(&globalFlags.DialTimeout, "dial-timeout", defaultDialTimeout, "dial timeout for client connections")