[backport 9.x] fix: return type of rateLimit

[MunifTanjim]

Backports https://github.com/fastify/fastify-rate-limit/pull/375 to 9.x

Checklist

• [ ] run npm run test and npm run benchmark
• [ ] tests and/or benchmarks are included
• [ ] documentation is changed or added
• [ ] commit message and code follows the Developer's Certification of Origin and the Code of conduct