Inconsistent output in documentation example: `num_classes` not displayed in `ClassLabel` output
In the documentation for [ClassLabel](https://huggingface.co/docs/datasets/v2.21.0/en/package_reference/main_classes#datasets.ClassLabel), there is an example of usage with the following code:

````
from datasets import Features
features = Features({'label': ClassLabel(num_classes=3, names=['bad', 'ok', 'good'])})
features
````

which expects to output (as stated in the documentation):

````
{'label': ClassLabel(num_classes=3, names=['bad', 'ok', 'good'], id=None)}
````

but it generates the following

````
{'label': ClassLabel(names=['bad', 'ok', 'good'], id=None)}
````

If my understanding is correct, this happens because although num_classes is used during the init of the object, it is afterward ignored:

https://github.com/huggingface/datasets/blob/be5cff059a2a5b89d7a97bc04739c4919ab8089f/src/datasets/features/features.py#L975

I would like to work on this issue if this is something needed 😄
