docs: fix example using db decorator on fastify instance (#4406)

* fix example using db decorator on fastify instance

I believe the current docs are a typo/broken example, unless that's an undocumented syntax.  This change adds an example for both returning the value since we're using async/await, along with an example for still using `reply.send()` and then `await`ing `reply`.

* remove semicolons

docs/Reference/Decorators.md

@@ -107,7 +107,13 @@ route [route](./Routes.md) handlers:
 fastify.decorate('db', new DbConnection())
 
 fastify.get('/', async function (request, reply) {
-  reply({hello: await this.db.query('world')})
+  // using return
+  return { hello: await this.db.query('world') }
+
+  // or
+  // using reply.send()
+  reply.send({ hello: await this.db.query('world') })
+  await reply
 })
 ```