Fix comment

common/errors.go

@@ -188,7 +188,7 @@ Theoretically it could be used to switch on the format string.
 	var err2 error = NewError("another kind of error")
 	...
 	// Error handling
-	switch err1.Data().(cmn.FmtError).Format {
+	switch err1.Data().(cmn.FmtError).Format() {
 		case "invalid username %v": ...
 		case "another kind of error": ...
 	    default: ...