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.