Fixed markdown formatting for file names in documentation

[AmanKhatri-ai]

Description of what this PR is changing or adding, and why: Fixed the incorrect markdown formatting in the document App Architecture/Architecture case study/Overview. Here's the link to the specified section- Architecture case study | Flutter.

Issues fixed by this PR (if any): Markdown formatting.

PRs or commits this PR depends on (if any):

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.

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[flutter-website-bot]

Visit the preview URL for this PR (updated for commit 15df28a):

https://flutter-docs-prod--pr11565-patch-1-w5k0ebyp.web.app

[parlough]

Nice catch, thanks so much for fixing this!

[AmanKhatri-ai]

Glad I could help. Thank you for reviewing and merging it!