fix syntax error

certbot/reverter.py

@@ -489,7 +489,7 @@ class Reverter(object):
 
         if not os.path.exists(changes_since_path):
             logger.info("Rollback checkpoint is empty (no changes made?)")
-            with open(self.config.changes_since_path) as f:
+            with open(changes_since_path, 'w') as f:
                 f.write("No changes\n")
 
         # Add title to self.config.in_progress_dir CHANGES_SINCE