Update AIShell recipe result

[pingfengluo]

1.update a slight better CER result from 5.12(50 epochs) to 4.94 (85 epochs, as conformer-ctc training) for confomer-mmi

2.fix train text (should only use train text in aishell_transcript_v0.8.txt, filter by uid in 'data/aishell/data_aishell/wav/train', as below shell script in prepare.sh, same way as KALDI)

    aishell_text=$dl_dir/aishell/data_aishell/transcript/aishell_transcript_v0.8.txt
    aishell_train_uid=$dl_dir/aishell/data_aishell/transcript/aishell_train_uid
    find data/aishell/data_aishell/wav/train -name "*.wav" | sed 's/\.wav//g' | awk -F '/' '{print $NF}' > $aishell_train_uid
    awk 'NR==FNR{uid[$1]=$1} NR!=FNR{if($1 in uid) print $0}' $aishell_train_uid $aishell_text | cut -d " " -f 2- > $lang_phone_dir/transcript_words.txt

[pingfengluo]

ready for review, thanks @csukuangfj

[csukuangfj]

Thanks! Merging.