fix reward in chess.json

[w9PcJLyb]

Because, at this moment, I can't initialize chess environment with steps if the game's result was a draw.

>>> kaggle_environments.make("chess", steps=game["steps"])

raise InvalidArgument(
kaggle_environments.errors.InvalidArgument: Default state generation failed for #0: 0.5 is not one of [-1, 0, 1]

Failed validating 'enum' in schema['properties']['reward']:
    {'description': '-1 = Lost, 0 = Draw/Ongoing, 1 = Won',
     'type': ['number', 'null'],
     'default': 0,
     'enum': [-1, 0, 1]}

On instance['reward']:
    0.5

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