Fix lexicon decoder docs (#2185)

Summary: Pull Request resolved: https://github.com/pytorch/audio/pull/2185

Reviewed By: hwangjeff, mthrok

Differential Revision: D33905767

Pulled By: carolineechen

fbshipit-source-id: 964576ab3f4a12b91fa3960b2aa2337239356513

docs/source/prototype.ctc_decoder.rst

@@ -6,11 +6,11 @@ torchaudio.prototype.ctc_decoder
 Decoder Class
 -------------
 
-KenLMLexiconDecoder
-~~~~~~~~~~~~~~~~~~~
+LexiconDecoder
+~~~~~~~~~~~~~~
 
 
-.. autoclass:: KenLMLexiconDecoder
+.. autoclass:: LexiconDecoder
 
   .. automethod:: __call__