docs: improve doctests for ndarray instances in ndarray/fill

[nightt5879]

Progresses #9329

Summary

• Replace the ndarray/fill usage doctest's explicit ndarray2array conversion with ndarray instance notation.
• Remove the now-unused @stdlib/ndarray/to-array import from that doctest.

Impact

This keeps the example focused on the returned ndarray while preserving the existing fill identity check.

Validation

• git diff --check - passed
• NODE_PATH=lib/node_modules node -e "const fill=require('@stdlib/ndarray/fill'); const zeros=require('@stdlib/ndarray/zeros'); const ndarray2array=require('@stdlib/ndarray/to-array'); const x=zeros([3,1,2], {'dtype':'float64'}); const y=fill(x,10.0); if (y !== x) { throw new Error('fill did not return input ndarray'); } const expected=[[[10,10]],[[10,10]],[[10,10]]]; const actual=ndarray2array(x); if (JSON.stringify(actual)!==JSON.stringify(expected)) { throw new Error('unexpected filled values: '+JSON.stringify(actual)); }" - passed

Checklist

• Read, understood, and followed the contributing guidelines

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Coverage Report

Package	Statements	Branches	Functions	Lines
ndarray/fill	$\color{green}116/116$ $\color{green}+100.00%$	$\color{green}7/7$ $\color{green}+100.00%$	$\color{green}1/1$ $\color{green}+100.00%$	$\color{green}116/116$ $\color{green}+100.00%$

The above coverage report was generated for the changes in this PR.

[Planeshifter]

⚠️ Issue Reference Review

An automated check found potentially unrelated issue/PR references in this PR:

Reference	Assessment	Reasoning
#9329	suspicious	#9329 is a tracking issue listing many packages to update with ndarray doctest improvements; this PR only updates ndarray/fill. Using "Closes" will auto-close the entire tracking issue when this PR is merged, which would prevent other contributors from seeing it and contributing additional package updates.

Why this matters: GitHub automatically closes issues referenced with closing keywords (Resolves, Closes, Fixes) when the PR is merged. Incorrect references can accidentally close unrelated issues.

What to do:

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• If the reference is incorrect, please update your PR description.

This assessment was generated by an AI model and is informational only.

---

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[stdlib-bot]

⚠️ Tracking Issue Closure Warning ⚠️

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Tracking issues should typically remain open until all related sub-issues are completed. GitHub automatically closes issues with such closing keywords when the PR is merged. For more information, see GitHub's documentation on using keywords in issues and pull requests.

Required action:
Use "Progresses" instead to reference the tracking issue without automatically closing it.

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