docs(readme): replace use of deprecated variadic listen()

README.md

@@ -47,7 +47,7 @@ fastify.get('/user/:id', function (req, reply) {
   })
 })
 
-fastify.listen(3000, err => {
+fastify.listen({ port: 3000 }, err => {
   if (err) throw err
 })
 ```