Update BuilderConfig to clean

When default is used it throws the following error:
ValueError: BuilderConfig 'default' not found. Available: ['clean']

Changing it to clean fixes the issue

README.md

@@ -127,7 +127,7 @@ pipe = pipeline(
     device=device,
 )
 
-dataset = load_dataset("distil-whisper/librispeech_long", "default", split="validation")
+dataset = load_dataset("distil-whisper/librispeech_long", "clean", split="validation")
 sample = dataset[0]["audio"]
 
 result = pipe(sample)