Update lm_train_tutorial.md

Yabin Li

2023-12-15 d2266616a142b7f57f29f1a840458a9865f953ac

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wget https://isv-data.oss-cn-hangzhou.aliyuncs.com/ics/MaaS/ASR/dep_libs/ffmpeg-N-111383-g20b8688092-linux64-gpl-shared.tar.xz
tar -xvf ffmpeg-N-111383-g20b8688092-linux64-gpl-shared.tar.xz