Fix LibriMix documentation (#2351)

Summary:
The `LibriMix` dataset is missing on the [documentation webpage](https://pytorch.org/audio/stable/datasets.html).

Pull Request resolved: https://github.com/pytorch/audio/pull/2351

Reviewed By: carolineechen

Differential Revision: D35926695

Pulled By: nateanl

fbshipit-source-id: 168aed3bb15510d1b1ec57d77727932e481aca48

docs/source/datasets.rst

@@ -50,6 +50,14 @@ GTZAN
   :special-members: __getitem__
 
 
+LibriMix
+~~~~~~~~
+
+.. autoclass:: LibriMix
+  :members:
+  :special-members: __getitem__
+
+
 LIBRISPEECH
 ~~~~~~~~~~~