Clarify number of samples taken in the resample function

In https://github.com/carpentries-incubator/machine-learning-novice-python/blob/gh-pages/_episodes/07-bootstrapping.md, the following chunk is used to resample the datasets for bootstrapping:

```
X_bs, y_bs = resample(x_train, y_train, replace=True)
```

The number of samples isn't specified in the function call, so it is unclear how many samples are being taken.

According to the documentation at https://scikit-learn.org/stable/modules/generated/sklearn.utils.resample.html the number of samples is specified in the `n_samples` argument:

> "n_samples int, default=None
Number of samples to generate. If left to None this is automatically set to the first dimension of the arrays. If replace is False it should not be larger than the length of arrays."

By default `resample` will use the length of the array as the number of samples. We should either: (1) note this default or (2) provide the `n_samples` argument.