9.4 锚框一节里的画出bbox 的注释书写错误

```
show_bboxes(fig.axes, boxes[250, 250, :, :] * bbox_scale,
            ['s=0.75, r=1', 's=0.75, r=2', 's=0.55, r=0.5', 's=0.5, r=1', 's=0.25, r=1'])
```

根据文章中的(s1, r1),(s1,r2), (s1,r3)第三个的注释应该是's=0.75, r=0.5', 如下所示：

```
show_bboxes(fig.axes, boxes[250, 250, :, :] * bbox_scale,
            ['s=0.75, r=1', 's=0.75, r=2', 's=0.75, r=0.5', 's=0.5, r=1', 's=0.25, r=1'])
```
