error in exercise 5.6.1

When visualising the model, it displays the original values, not the modelled values:

```
ggplot(augment(lme_yield), aes(x = irrigation, y = yield, colour = variety)) +
  geom_point(position = position_dodge(width = 0.5))
```

should be

```
ggplot(augment(lme_yield), aes(x = irrigation, y = .fitted, colour = variety)) +
  geom_point(position = position_dodge(width = 0.5))
```