Fix broken link in README.md (#436)

Summary:
https://einstein.ai/research/the-wikitext-long-term-dependency-language-modeling-dataset is not longer valid, redirects to a blog post listing page.
Pull Request resolved: https://github.com/pytorch/fairseq/pull/436

Differential Revision: D13607961

Pulled By: myleott

fbshipit-source-id: 1a1074ffcbc454e29bc9d5aed84fdf2089a224bc