docs(corelib): fix three doc-comment examples

[orizi]

Summary

Fixes three small documentation bugs in the corelib:

• ops/function.cairo: Corrects a misplaced code fence in the FnOnce doc comment. The opening ``` was placed before the # Examples header instead of after it, and the closing ``` was missing entirely.
• option.cairo: Adds a missing semicolon at the end of the let o: Option<u8> = 67_u8.into() statement in the TIntoOption example.
• zeroable.cairo: Fixes the type used in the Zeroable::zero example from i32 to u32, matching the trait's typical usage with unsigned integers.

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Type of change

Please check one:

• Bug fix (fixes incorrect behavior)
• New feature
• Performance improvement
• Documentation change with concrete technical impact
• Style, wording, formatting, or typo-only change

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Why is this change needed?

The FnOnce doc comment had a malformed code block that would render incorrectly in generated documentation. The option.cairo example had a syntax error (missing semicolon) that would fail if executed. The zeroable.cairo example referenced i32, which is inconsistent with the unsigned integer context typically associated with Zeroable.

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What was the behavior or documentation before?

• FnOnce: The ``` opening fence appeared before # Examples, and the closing fence was absent, causing the entire example to render as a broken code block.
• TIntoOption: The example statement 67_u8.into() lacked a terminating semicolon.
• Zeroable::zero: The example used Zeroable::<i32>::zero().

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What is the behavior or documentation after?

• FnOnce: The code fence is correctly placed after # Examples, and the closing fence is present.
• TIntoOption: The example statement is syntactically valid with a semicolon.
• Zeroable::zero: The example uses Zeroable::<u32>::zero().

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Related issue or discussion (if any)

None.

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Additional context

None.

[reviewable-StarkWare]

This change is 

[orizi]

• docs(corelib): fix three doc-comment examples #10204 👈 (View in Graphite)
• main

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PR Summary

Low Risk
Comment-only edits with no runtime or API behavior changes.

Overview
Fixes documentation-only issues in three corelib doc-comment examples so they render and read correctly.

In ops/function.cairo, the FnOnce example block is repaired: the opening fence now follows # Examples, and a closing fence is added so the snippet is a proper fenced block.

In option.cairo, the TIntoOption example adds a semicolon after 67_u8.into().

In zeroable.cairo, the Zeroable::zero example switches from i32 to u32 in the type argument.

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