Chapter 3: Adding axis labels for function plot_confusion_matrix()

Hi ageron,
I had a lot of fun reading the chapter 3, and I figured it would be nice to have x and y axis labeled in the function plot_confusion_matrix(). It would make the graph more intuitive.
Code below could be added to the function:

```python
plt.ylabel('actual')
plt.xlabel('predicted')
ax.xaxis.set_label_position('top')
```