Update README.md

[thebspin]

I changed const fastify = require('fastify') in the examples to const fastify = require('fastify')() since that seems the (new?) and correct way to work.
If i don't use that i get an error saying TypeError: fastify.register is not a function

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, good spot!

[thebspin]

lgtm, good spot!

Thanks!