[egs] Training amount of ivector in librispeech recipe

https://github.com/kaldi-asr/kaldi/blob/9d235864c3105c3b72feb9f19a219dbae08b3a41/egs/librispeech/s5/local/nnet3/run_ivector_common.sh#L86

According to the comment above this data subset script, 200 hours of data will extracted. However, the source data ```data/${train_set}_sp_hires``` is speed perturbed, so there should be around 3000 hours in ```data/${train_set}_sp_hires```, therefore 600 hours to train an ivector?

Start at egs/librispeech/s5/local/nnet3/run_ivector_common.sh around line 86. Check the comment and the data/${train_set}_sp_hires source, including the effect of speed perturbation, to determine whether the stated training amount is correct. Done means resolving the discrepancy and identifying whether the comment or script needs correction.

Written by the indexing model from the issue text.