Fix fluent transition changeset example

[SandroMaglione]

Summary

• wrap the fluent transition migration example in a complete object literal so Markdown formatting preserves valid, readable TypeScript

Changeset

• Added or updated for a library or package-metadata change
• Not required because this PR does not change src/ or package.json

Validation

• pnpm check
• Relevant example checks, when examples changed
• Automated type-performance measurement passed or was not required
• Automated runtime- and memory-performance measurement passed or was not required

Focused validation: pnpm format:check and pnpm docs:site:check.