Fix doc warning (#2627)

Summary:
Resolves the following warning

```
/torchaudio/docs/source/transforms.rst:94: WARNING: Title underline too short.

:hidden:`Loudness`
-----------------
```

Pull Request resolved: https://github.com/pytorch/audio/pull/2627

Reviewed By: carolineechen

Differential Revision: D38814802

Pulled By: mthrok

fbshipit-source-id: 5dfaf2d7bae22dba0f4a14f04ca63f28d6b2a749

docs/source/transforms.rst

@@ -91,7 +91,7 @@ Transforms are common audio transforms. They can be chained together using :clas
   .. automethod:: forward
 
 :hidden:`Loudness`
------------------
+------------------
 
 .. autoclass:: Loudness