Add missing parentheses in regex expression (#567)

Summary:
The regex pattern without parentheses is not correct. The checkpoints are not sorted in descending order
Pull Request resolved: https://github.com/pytorch/fairseq/pull/567

Differential Revision: D14404380

Pulled By: myleott

fbshipit-source-id: 98cd0cfa8c92b78a03ffbb94840bc0f7a118eca1