Update README.md (facebookresearch#1767)

Summary:
# Before submitting

- [ ] Was this discussed/approved via a Github issue? (no need for typos, doc improvements)
- [ ] Did you read the [contributor guideline](https://github.com/pytorch/fairseq/blob/master/CONTRIBUTING.md)?
- [ ] Did you make sure to update the docs?
- [ ] Did you write any new necessary tests?

## What does this PR do?
Fixes a link in a README.

## PR review
Anyone in the community is free to review the PR once the tests have passed.
If we didn't discuss your PR in Github issues there's a high chance it will not be merged.

## Did you have fun?
Make sure you had fun coding �
Pull Request resolved: facebookresearch#1767

Reviewed By: ngoyal2707

Differential Revision: D20248559

Pulled By: myleott

fbshipit-source-id: 0becc73d8bca337286faa40ce100a94cd319fec9

examples/bart/README.md

@@ -200,7 +200,7 @@ files2rouge test.hypo.tokenized test.hypo.target
 ## Finetuning
 
 - [Finetuning on GLUE](README.glue.md)
-- [Finetuning on CNN-DM](README.cnn.md)
+- [Finetuning on CNN-DM](README.summarization.md)
 
 ## Citation