fixed a bug in preprocess glue dataset dev filename (#1270)

Summary:
removed redundant quotes in the filename assigned for dev dataset for GLUE tasks
Pull Request resolved: https://github.com/pytorch/fairseq/pull/1270

Differential Revision: D18013071

fbshipit-source-id: 35f00162e117c6584dc859f760503ca32dcb706e

examples/roberta/preprocess_GLUE_tasks.sh

@@ -173,7 +173,7 @@ do
     fairseq-preprocess \
       --only-source \
       --trainpref "$TASK_DATA_FOLDER/processed/train.label" \
-      --validpref "${DEVPREF//LANG/'label'}" \
+      --validpref "${DEVPREF//LANG/label}" \
       --destdir "$TASK-bin/label" \
       --workers 60;
   else