docs: improve doctests for ndarray instances in ndarray/flatten

[ShreelaxmiHegde]

Progresses #9329.

Description

What is the purpose of this pull request?

This pull request:

• Updates the examples for ndarray/flatten package, by migrating to using ndarray instance notation.

Related Issues

Does this pull request have any related issues?

This pull request has the following related issues:

• [RFC]: Improve doctests for ndarray instances in documentation examples (tracking issue) #9329

Questions

Any questions for reviewers of this pull request?

No.

Other

Any other information relevant to this pull request? This may include screenshots, references, and/or implementation notes.

No.

Checklist

Please ensure the following tasks are completed before submitting this pull request.

• Read, understood, and followed the contributing guidelines.

AI Assistance

When authoring the changes proposed in this PR, did you use any kind of AI assistance?

• Yes
• No

{{TODO: add disclosure if applicable}}

---

@stdlib-js/reviewers

[stdlib-bot]

Coverage Report

Package	Statements	Branches	Functions	Lines
ndarray/flatten	$\color{green}360/360$ $\color{green}+100.00%$	$\color{green}30/30$ $\color{green}+100.00%$	$\color{green}1/1$ $\color{green}+100.00%$	$\color{green}360/360$ $\color{green}+100.00%$

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

[Planeshifter]

LGTM!

[stdlib-bot]

PR Commit Message

PR-URL: 

Reviewed-by: Philipp Burckhardt <pburckhardt@outlook.com>

Please review the above commit message and make any necessary adjustments.