fix import bug in global_stats.py (#2858)

Summary:
This code was added by
https://github.com/pytorch/audio/commit/4d0095a528412cfec2a549204fc01d9ebb15df7a

Seems that the original code had a typo?

Pull Request resolved: https://github.com/pytorch/audio/pull/2858

Test Plan:
```
// the import of `mustc` now succeeds, previously crashed
python examples/asr/emformer_rnnt/global_stats.py --model-type librispeech --dataset-path /home/vasiliy/local/librispeech/
```

Reviewed By: carolineechen

Differential Revision: D41355663

Pulled By: nateanl

fbshipit-source-id: 92507e529d41b984b9dd400ad24a55d130372b7d

examples/asr/emformer_rnnt/global_stats.py

@@ -19,7 +19,7 @@ from common import (
     piecewise_linear_log,
     spectrogram_transform,
 )
-from must.dataset import MUSTC
+from mustc.dataset import MUSTC
 
 
 logger = logging.getLogger()