Documentation: fix link to language model readme (#600)
Documentation: fix link to language model readme (#600)

Summary:
Hi,

currently, the link to the language model readme is broken on the `examples/language_model/transformer_lm` page.

This PR fixes the link :)
Pull Request resolved: https://github.com/pytorch/fairseq/pull/600

Differential Revision: D14680985

Pulled By: myleott

fbshipit-source-id: 62291efbf4ece2af54fae45c408c2759863f9847

pytorch/fairseq,pytorch/fairseq,pytorch/fairseq