Error in examples/demo.ipynb testing on a batch

In "Testing on a batch":

```python
fig = plt.figure(figsize=(10, 5))
for i, pred in enumerate(preds):
plt.subplot(1, 2, i + 1)
plt.imshow(frames[1])
plt.title(f'frame[{i}]')
for detection in pred:
plt.scatter(detection[:,0], detection[:,1], 2)
```
the 2nd loop is redundant, need to plot `pred[:, 0], pred[:, 1]` only.

Open examples/demo.ipynb, locate the 'Testing on a batch' section. The issue points to a redundant loop: instead of iterating over detections, plot pred[:,0] and pred[:,1] directly. Check the surrounding code to understand the data shape, then correct the plotting. Run the notebook cell to verify the fix produces the expected visualization.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.