Fix doc in torchaudio.backend (#2781)

Summary:
address https://github.com/pytorch/audio/issues/2780

Pull Request resolved: https://github.com/pytorch/audio/pull/2781

Reviewed By: carolineechen, mthrok

Differential Revision: D40556794

Pulled By: nateanl

fbshipit-source-id: b24912489d41e5663b4b4dcfb8be743fb962097e

docs/source/backend.rst

@@ -10,7 +10,7 @@ Overview
 
 :mod:`torchaudio.backend` module provides implementations for audio file I/O functionalities, which are ``torchaudio.info``, ``torchaudio.load``, and ``torchaudio.save``.
 
-There are currently four implementations available.
+There are currently two implementations available.
 
 * :py:mod:`"sox_io" <torchaudio.backends.sox_io_backend>` (default on Linux/macOS)
 * :py:mod:`"soundfile" <torchaudio.backends.soundfile_backend>` (default on Windows)