fix a typoe

In UNIT4 : Pretrained models for audio classification
We’ll load an official [Audio Spectrogram Transformer](https://huggingface.co/docs/transformers/model_doc/audio-spectrogram-transformer) checkpoint fine-tuned on the Speech Commands dataset, under the namespace ["MIT/ast-finetuned-speech-commands-v2"](https://huggingface.co/MIT/ast-finetuned-speech-commands-v2):

Copied
classifier = pipeline(
"audio-classification", model="MIT/ast-finetuned-speech-commands-v2"
)
classifier(sample["audio"])
Fix it to be classifier(sample["audio"]["array"])
I don't know how to make a pull request yet! :)

Start in UNIT4 at the “Pretrained models for audio classification” example and inspect the shown classifier call. Update the audio argument to use the sample’s array, then verify the lesson displays the corrected example.

Written by the indexing model from the issue text.