docs: update link to fastify 1.x compatible version

[is2ei]

This PR updates the link to fastify 1.x compatible version of this plugin since https://github.com/fastify/fastify-cors/tree/1.x is dead.

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

[Fdawgs]

Thanks @is2ei