Fix code quality bug in riddle_sense dataset
COLLABORATOR
To give more context: https://github.com/psf/black/issues/318. `black` doesn't treat this as a bug, but `flake8` does. 

2021-11-04T17:45:54Z
https://github.com/huggingface/datasets/issues/3217#issuecomment-961277312
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2021-11-04T17:45:54Z
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## Describe the bug
```
datasets/riddle_sense/riddle_sense.py:36:21: W291 trailing whitespace
```