Fix ast.metal_kernel typo in the custom Metal kernels guide

[ayaangazali]

Proposed changes

The custom Metal kernels guide refers to ast.metal_kernel.__call__, which does not exist, so the link is dead:

docs/src/dev/custom_metal_kernels.rst:95: WARNING: py:func reference target not found:
ast.metal_kernel.__call__ [ref.func]

Looks like a typo for fast.metal_kernel, which is how the same page refers to it in five other places.

While fixing the name I also adjusted the wording, because verbose is not an argument to fast.metal_kernel itself. It is passed when calling the kernel that metal_kernel returns, which is what the example further up the page does:

kernel = mx.fast.metal_kernel(...)
outputs = kernel(
    inputs=[a],
    grid=(a.size, 1, 1),
    threadgroup=(256, 1, 1),
    verbose=True,
)

So the sentence now says "when calling the kernel returned by :func:fast.metal_kernel" rather than pointing at the factory function.

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)

(docs only; the page now builds with no unresolved references)

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small disclosure, same as my other PRs: freshman contributor and Claude Code helps me sweep, but i built the docs in nitpick mode to find this, then checked the metal_kernel docstring to confirm where verbose actually belongs before rewording rather than just swapping the typo.