Wrong output on chapter4/fine-tuning

Hi👋,
I think the output should be:
```
DatasetDict({
train: Dataset({
features: ['genre', 'input_values', 'attention_mask'],
num_rows: 899
})
test: Dataset({
features: ['genre', 'input_values', 'attention_mask'],
num_rows: 100
})
})
```

Instead of

https://github.com/huggingface/audio-transformers-course/blob/c3a8701dd376ad7c6b1e0a4a8c6970ccc2d60cb9/chapters/en/chapter4/fine-tuning.mdx#L321-L336

Since `return_attention_mask=True` in the `feature_extractor`. Is this the case?