docs(readme): fix route config link (#274)

The route config link still points to document for fastify 2.1.x.

This MR modifies the link to 4.8.x (since there is no link for major version `4.x`). It's slightly better than just pointing to `latest` as this plugin might not be compatible with the future fastify version (e.g. v5) right away.

Author: wtchangdm

README.md

@@ -145,7 +145,7 @@ fastify.addContentTypeParser('application/xml', (req, done) => {
 ### config
 
 An object accessible within the `preHandler` via `reply.context.config`.
-See [Config](https://www.fastify.io/docs/v2.1.x/Routes/#config) in the Fastify
+See [Config](https://www.fastify.io/docs/v4.8.x/Reference/Routes/#config) in the Fastify
 documentation for information on this option. Note: this is merged with other
 configuration passed to the route.