Merge pull request #48047 from yan234280533/modify_grammar_1

Automatic merge from submit-queue

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

What this PR does / why we need it:
We found that the error message of kubectl secret command when the the envFile path is not an file path
is inaccurate and the style is different with which in  kubectl configmap command. We modified “must be a file” to "env secret file cannot be a directory" 
Special notes for your reviewer:
None

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 {