modify the meassage in kubectl secret command when the envFile path is not an file path

pkg/kubectl/secret.go

@@ -217,7 +217,7 @@ func handleFromEnvFileSource(secret *api.Secret, envFileSource string) error {
 		}
 	}
 	if info.IsDir() {
-		return fmt.Errorf("must be a file")
+		return fmt.Errorf("env secret file cannot be a directory")
 	}
 
 	return addFromEnvFile(envFileSource, func(key, value string) error {