[Fix doc example] UniSpeechSatForPreTraining
# What does this PR do?

In `UniSpeechSatForPreTraining`, this doc example fails

```
>>> feature_extractor = UniSpeechSatFeatureEncoder.from_pretrained("patrickvonplaten/unispeech_sat-base")
>>> model = UniSpeechSatForPreTraining.from_pretrained("patrickvonplaten/unispeech_sat-base")
```
(can't find this checkpoint on the Hub)

This PR changes it to

```
>>> feature_extractor = Wav2Vec2FeatureExtractor.from_pretrained("microsoft/unispeech-sat-base")
>>> model = UniSpeechSatForPreTraining.from_pretrained("microsoft/unispeech-sat-base")
```

## Who can review

@patrickvonplaten 