fix(docs): list example and index option

[ziimakc]

Witout index=false option fastify-static will return 404 as it will look for index.hml instead returning list of files.

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

[mcollina]

lgtm