Import error in background_segmenter tutorial

Uncaught SyntaxError: The requested module 'https://cdn.jsdelivr.net/npm/@mediapipe/tasks-vision' does not provide an export named 'default' (at index.js:2:8)

Fix:
```javascript
import * as vision from "https://cdn.jsdelivr.net/npm/@mediapipe/tasks-vision";
```
https://github.com/google-ai-edge/mediapipe-samples/blob/main/tutorials/background_segmenter/code/index.js#L2