Could not connect to websocket error when using subscription in graphql example source code

I'm got this error with response when calling subscription in **graphql example** source code. Please check the error description and image bellow.
```
{
"error": "Could not connect to websocket endpoint ws://localhost:3000/subscriptions. Please check if the endpoint url is correct."
}
```

STEP TO REPRODUCE.

```
Step 1: Clone the loopback-next source code.
Step 2: Run npm install in the root of source code (not in the example directory)
Step 3: npm start and check the error when calling subscription.
```

System information
```
OS: Windows 10
CPU: AMD Ryzen 5
```

Start by following the issue's reproduction steps in the graphql example and inspect how the subscription WebSocket endpoint is configured. Compare the configured endpoint with ws://localhost:3000/subscriptions and verify the subscription connects successfully; the issue is done when the example no longer reports the connection error.

Written by the indexing model from the issue text.