Fix link to RACE fine-tuning instructions.

Summary: Pull Request resolved: https://github.com/pytorch/fairseq/pull/1125

Differential Revision: D17431557

Pulled By: myleott

fbshipit-source-id: f712e5355d8dbb0a8f1170674d62e2b6880295b4

examples/roberta/commonsense_qa/README.md

 # Finetuning RoBERTa on Commonsense QA
 
-We follow a similar approach to [finetuning RACE](README.race.md). Specifically
+We follow a similar approach to [finetuning RACE](../README.race.md). Specifically
 for each question we construct five inputs, one for each of the five candidate
 answer choices. Each input is constructed by concatenating the question and
 candidate answer. We then encode each input and pass the resulting "[CLS]"