Merge pull request #2153 from PaddlePaddle/Jackwaterveg-patch-1

[Doc] fix doc

examples/aishell/asr1/README.md

 # Transformer/Conformer ASR with Aishell
-This example contains code used to train a Transformer or [Conformer](http://arxiv.org/abs/2008.03802) model with [Aishell dataset](http://www.openslr.org/resources/33)
+This example contains code used to train a [u2](https://arxiv.org/pdf/2012.05481.pdf) model (Transformer or [Conformer](https://arxiv.org/pdf/2005.08100.pdf) model) with [Aishell dataset](http://www.openslr.org/resources/33)
 ## Overview
 All the scripts you need are in `run.sh`. There are several stages in `run.sh`, and each stage has its function.
 | Stage | Function                                                     |

examples/librispeech/asr1/README.md

 # Transformer/Conformer ASR with Librispeech
-This example contains code used to train a Transformer or [Conformer](http://arxiv.org/abs/2008.03802) model with [Librispeech dataset](http://www.openslr.org/resources/12)
+This example contains code used to train [u2](https://arxiv.org/pdf/2012.05481.pdf) model (Transformer or [Conformer](https://arxiv.org/pdf/2005.08100.pdf) model) with [Librispeech dataset](http://www.openslr.org/resources/12)
 ## Overview
 All the scripts you need are in `run.sh`. There are several stages in `run.sh`, and each stage has its function.
 | Stage | Function                                                     |

examples/librispeech/asr2/README.md

 # Transformer/Conformer ASR with Librispeech ASR2
 
-This example contains code used to train a Transformer or [Conformer](http://arxiv.org/abs/2008.03802) model with [Librispeech dataset](http://www.openslr.org/resources/12) and use some functions in kaldi.
+This example contains code used to train a [u2](https://arxiv.org/pdf/2012.05481.pdf) model (Transformer or [Conformer](https://arxiv.org/pdf/2005.08100.pdf) model) with [Librispeech dataset](http://www.openslr.org/resources/12) and use some functions in kaldi.
 
 To use this example, you need to install Kaldi first.
 

examples/tiny/asr1/README.md

 # Transformer/Conformer ASR with Tiny
-This example contains code used to train a Transformer or [Conformer](http://arxiv.org/abs/2008.03802) model  Tiny dataset(a part of [[Librispeech dataset](http://www.openslr.org/resources/12)](http://www.openslr.org/resources/33))
+This example contains code used to train a [u2](https://arxiv.org/pdf/2012.05481.pdf) model (Transformer or [Conformer](https://arxiv.org/pdf/2005.08100.pdf) model) with Tiny dataset(a part of [[Librispeech dataset](http://www.openslr.org/resources/12)](http://www.openslr.org/resources/33))
 ## Overview
 All the scripts you need are in `run.sh`. There are several stages in `run.sh`, and each stage has its function.
 | Stage | Function                                                     |