Fix build doc (#3435)

Summary: Pull Request resolved: https://github.com/pytorch/audio/pull/3435

Reviewed By: nateanl

Differential Revision: D46659362

Pulled By: mthrok

fbshipit-source-id: ffa033ad6759de6fd958b63ac51a4a1153ffb45d

torchaudio/functional/functional.py

@@ -1305,7 +1305,7 @@ def apply_codec(
 
     .. devices:: CPU
 
-    .. warnings::
+    .. warning::
 
        This function has been deprecated.
        Please migrate to :py:class:`torchaudio.io.AudioEffector`, which works on all platforms,