Update 3.routing.md

After some confusion trying to identify which command to use to fetch an API using only Nitro, I was not sure if the same command on Nuxt would work here.

To avoid this confusion and to make it clear to anybody that `$fetch` is the command to use I believe this should be in the documentation

docs/content/1.guide/3.routing.md

@@ -35,6 +35,8 @@ export default eventHandler(() => {
 })
 ```
 
+You can now universally call this API using `await $fetch('/api/hello')`.
+
 ### Route with params
 
 ```js