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

examples/language_model/transformer_lm/README.md

@@ -9,7 +9,7 @@ Adaptive Inputs <br> ([Baevski and Auli, 2018](https://arxiv.org/abs/1809.10853)
 
 ## Example usage
 
-See the [language modeling README](../language_model/README.md) for instructions on reproducing results for WikiText-103
+See the [language modeling README](../README.md) for instructions on reproducing results for WikiText-103
 using the `transformer_lm_wiki103` model architecture.
 
 ## Citation