docs: fix error message (#197)

README.md

@@ -225,7 +225,7 @@ from existing notebooks. This invocation operates on all ipynb files in the repo
         if old_len != len(blob.data):
             print(change.blob_id, change.filename, old_len, len(blob.data))
     except NotJSONError as e:
-         print("ERROR", type(e), change.blob_id, filename)
+         print("ERROR", type(e), change.blob_id, change.filename)
     '
 ```