Fix default value for --context-size in icefall. (#1538)

egs/aishell/ASR/pruned_transducer_stateless7_streaming/decode.py

@@ -250,7 +250,7 @@ def get_parser():
  parser.add_argument(
  "--context-size",
  type=int,
- default=1,
+ default=2,
  help="The context size in the decoder. 1 means bigram; 2 means tri-gram",
  )
  parser.add_argument(