[doc] fix: fix typo in docs/preparation/prepare_data.rst (#2957)

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I fixed the typo from RewardModule to RewardModel in
docs/preparation/prepare_data.rst

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Author: nariaki3551

docs/preparation/prepare_data.rst

@@ -73,7 +73,7 @@ In the ``make_map_fn``, each data field should consist of the following
 5 fields:
 
 1. ``data_source``: The name of the dataset. To index the corresponding
-   reward function in the ``RewardModule``
+   reward function in the ``RewardModel``
 2. ``prompt``: This field should be constructed in the format of
    huggingface chat_template. The tokenizer in ``RLHFDataset`` will
    apply chat template and tokenize the prompt.