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

examples/language_model/README.md

@@ -4,7 +4,7 @@ These scripts provide an example of pre-processing data for the Language Modelin
 
 # prepare-wikitext-103.sh
 
-Provides an example of pre-processing for [WikiText-103 language modeling task](https://einstein.ai/research/the-wikitext-long-term-dependency-language-modeling-dataset):
+Provides an example of pre-processing for [WikiText-103 language modeling task](https://www.salesforce.com/products/einstein/ai-research/the-wikitext-dependency-language-modeling-dataset/):
 
 Example usage:
 ```