Readme: Fix link to mBART documentation (#1789)

Summary:
Hi,

this PR updates the link to mBART documentation in main readme.
Pull Request resolved: #1789

Differential Revision: D20322673

Pulled By: myleott

fbshipit-source-id: b59c94f49176ba5bbd664791818b5b8ce7402698

README.md

@@ -51,7 +51,7 @@ Fairseq provides reference implementations of various sequence-to-sequence model
   - [RoBERTa: A Robustly Optimized BERT Pretraining Approach (Liu et al., 2019)](examples/roberta/README.md)
   - [Facebook FAIR's WMT19 News Translation Task Submission (Ng et al., 2019)](examples/wmt19/README.md)
   - [Jointly Learning to Align and Translate with Transformer Models (Garg et al., 2019)](examples/joint_alignment_translation/README.md )
-  - [Multilingual Denoising Pre-training for Neural Machine Translation (Liu et at., 2020)] (examples/mbart/README.md)
+  - [Multilingual Denoising Pre-training for Neural Machine Translation (Liu et at., 2020)](examples/mbart/README.md)
 - **Non-autoregressive Transformers**
   - Non-Autoregressive Neural Machine Translation (Gu et al., 2017)
   - Deterministic Non-Autoregressive Neural Sequence Modeling by Iterative Refinement (Lee et al. 2018)