Use unicode box-drawing characters for code organization

[mateusfccp]

Description of what this PR is changing or adding, and why:

This PR changes the example of code organization to use unicode box-drawing characters instead of | and -.

It makes the example more readable.

Issues fixed by this PR (if any): none.

PRs or commits this PR depends on (if any): none.

Presubmit checklist

• This PR is marked as draft with an explanation if not meant to land until a future stable release.
• This PR doesn’t contain automatically generated corrections (Grammarly or similar).
• This PR follows the Google Developer Documentation Style Guidelines — for example, it doesn’t use i.e. or e.g., and it avoids I and we (first person).
• This PR uses semantic line breaks of 80 characters or fewer.

[sfshaza2]

/gcbrun

[flutter-website-bot]

Visit the preview URL for this PR (updated for commit 2b3e34c):

https://flutter-docs-prod--pr11799-patch-1-czakbyer.web.app

[sfshaza2]

Yes, that is easier to read. Thx!

[sfshaza2]

Definitely easier to read. thx!