fixing flake8

Author: antgonza

qiita_pet/handlers/study_handlers/description_handlers.py

@@ -716,7 +716,7 @@ def delete_preprocessed_data(self, study, user, callback):
             PreprocessedData.delete(ppd_id)
             msg = ("Preprocessed data %d has been deleted" % ppd_id)
             msg_level = "success"
-            pd_id = None
+            ppd_id = None
         except Exception as e:
             msg = ("Couldn't remove preprocessed data %d: %s" %
                    (ppd_id, str(e)))