[Docs] Correct Fly.io sandbox requirement grammar

[roomote-community]

​Opened on behalf of @mrubens. Follow up by mentioning @roomote, in the web UI, or in Discord.

Related issue

No linked issue. This is internal documentation maintenance from a repository typo scan.

Why this PR exists

• A maintainer explicitly invited this PR in the linked issue or discussion
• I am a maintainer / this is internal Roomote work

The Fly.io deployment overview used a singular verb with the plural subject "hosted sandboxes."

What changed

Correct the subject-verb agreement so the deployment guide states that hosted sandboxes are required when a Docker socket is unavailable. This is documentation-only and does not change runtime behavior.

How it was tested

• git diff --check
• Repository pre-push checks: oxlint, residual lint, fast type checks, and knip

Checklist

• The PR title follows the repo convention: [Fix], [Feat], [Improve], [Refactor], [Docs], or [Chore] followed by a user-facing description
• This PR is small and scoped to one change
• pnpm lint and pnpm check-types pass locally
• I added tests or included a clear manual validation note above
• I removed secrets, tokens, private keys, and customer data from code, logs, and screenshots
• If this change should appear in the changelog, I ran pnpm changeset

[roomote-community]

No code issues found. See task

_{Reviewed e2d10ea}