Fix lm used for ctc decoder example (#2235)

Summary:
LM in example script was unintentionally changed to None when adding no LM support previously. this changes it back and is consistent with the WERs listed in the readme

Pull Request resolved: https://github.com/pytorch/audio/pull/2235

Reviewed By: nateanl

Differential Revision: D34273042

Pulled By: carolineechen

fbshipit-source-id: 824b1ce18195e39dc534b2ec9c5312bbe3bb1812

examples/asr/librispeech_ctc_decoder/inference.py

@@ -34,7 +34,7 @@ def run_inference(args):
     decoder = lexicon_decoder(
         lexicon=lexicon_file,
         tokens=tokens,
-        lm=None,
+        lm=kenlm_file,
         nbest=1,
         beam_size=1500,
         beam_size_token=None,