Correct grammatical typo in label smoothing ValueError - #3641

Proposed changes

This PR fixes a minor grammatical typo in the ValueError message raised when the label_smoothing parameter in cross_entropy is out of the valid [0, 1) range.

Before: "Label smoothing must in [0, 1), got {label_smoothing}."
After: "Label smoothing must be in [0, 1), got {label_smoothing}."

While simple, error messages are a primary interface for developers using MLX. Grammatically correct and clear error messages:

1. Enhance Developer Experience (DX): Prevent momentary cognitive overhead or confusion when a user is diagnosing parameter issues during training configuration.
2. Polished API Presentation: Keep the library's developer-facing interface clean and professional.

Checklist

Put an x in the boxes that apply.

• I have read the CONTRIBUTING document
• I have run pre-commit run --all-files to format my code / installed pre-commit prior to committing changes
• I have added tests that prove my fix is effective or that my feature works
• I have updated the necessary documentation (if needed)