Typo in mlx tensor support

[pcuenca]

What does this PR do?

#29406 recently added MLX tensor support to BatchEncoding. However, I think there's a typo in _is_mlx when importing the mlx.core module.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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• Did you write any new necessary tests?

Who can review?

cc @Y4hL for verification

[tidely]

Yeah, it's a typo

[HuggingFaceDocBuilderDev]

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[amyeroberts]

Thank for fixing!

Reviewing in place of @ArthurZucker as I'm watching the repo now :)

[pcuenca]

Merged main to pull 4ed9ae6 and fix the failing test.

[pcuenca]

Feel free to merge whenever (I can't do it) :)