Fix data path in test_continuing_prefix_trainer_mismatch

[GaetanLepage]

Context: bumping the tokenizer version in nixpkgs.
NixOS/nixpkgs#389517

I observed the following test failure:

=================================== FAILURES ===================================
_____________ TestUnigram.test_continuing_prefix_trainer_mismatch ______________

self = <tests.bindings.test_trainers.TestUnigram object at 0x7ffff4ce1d30>

    def test_continuing_prefix_trainer_mismatch(self):
        UNK = "[UNK]"
        special_tokens = [UNK]
        tokenizer = Tokenizer(models.BPE(unk_token=UNK, continuing_subword_prefix="##"))
        trainer = trainers.BpeTrainer(special_tokens=special_tokens)
        tokenizer.pre_tokenizer = pre_tokenizers.Sequence(
            [pre_tokenizers.Whitespace(), pre_tokenizers.Digits(individual_digits=True)]
        )
>       tokenizer.train(files=["data/big.txt"], trainer=trainer)
E       Exception: No such file or directory (os error 2)

tests/bindings/test_trainers.py:298: Exception

This PR provides a patch that fixes the test.
Feel free to propose an alternative solution if needed.