Typo in the figure API (fig.suptitle)

The following is from https://matplotlib.org/tutorials/introductory/usage.html#sphx-glr-tutorials-introductory-usage-py : 

```
fig = plt.figure()  # an empty figure with no axes
fig.suptitle('No axes on this figure')  # Add a title so we know which it is
```

The API should be `fig.subtitle` instead of `fig.suptitle` because 'suptitle' is not an english word.