Update data_utils.py for (#598)
Update data_utils.py for (#598)

Summary:
Correcting the syntax error in assert function cause of a character before error message.

Assertion and the code is working fine now, Tested with wmt-ende task.
Pull Request resolved: https://github.com/pytorch/fairseq/pull/598

Differential Revision: D14712846

Pulled By: myleott

fbshipit-source-id: 3f708aa2362ceecba19174750f9ffc9238537512

pytorch/fairseq,pytorch/fairseq,pytorch/fairseq