fix: return type of rateLimit

[MunifTanjim]

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

[MunifTanjim]

@jsumners @Uzlopak Can I get a review on this one? Without these changes we get TypeScript error when trying to use this package.

[jsumners]

I cannot read TypeScript. Please do not tag me on any TypeScript changes.