Fix docs: replace nonexistent mx.random.randn with mx.random.normal

[Satyam12singh]

Proposed changes

The documentation under "Boolean masks follow NumPy semantics" uses the example:

a[mx.random.randn(10, 10) > 0.0] = 0

However, MLX does not implement mx.random.randn. Running this code produces:

AttributeError: module 'mlx.core.random' has no attribute 'randn'

This PR replaces the invalid call with the correct MLX API:

mx.random.normal((10, 10))

This makes the documentation runnable and consistent with the current MLX API.

Checklist

Put an x in the boxes that apply.

• [ x] 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
• [ x] I have updated the necessary documentation (if needed)

[angeloskath]

Thanks!