Involve dev dataset to build vocabulary in the aishell example

deep_speech_2/examples/aishell/run_data.sh

@@ -17,7 +17,7 @@ fi
 python tools/build_vocab.py \
 --count_threshold=0 \
 --vocab_path='data/aishell/vocab.txt' \
---manifest_paths='data/aishell/manifest.train'
+--manifest_paths 'data/aishell/manifest.train' 'data/aishell/manifest.dev'
 
 if [ $? -ne 0 ]; then
     echo "Build vocabulary failed. Terminated."