docs: use `defineEventHandler()` to avoid warnings (#18557)

Avoid the warning:

> [h3] Implicit event handler conversion is deprecated. Use `eventHandler()` or `fromNodeMiddleware()` to define event handlers.

docs/1.getting-started/6.data-fetching.md

@@ -83,10 +83,10 @@ In brief, `useFetch` receives a URL and gets that data, whereas `useAsyncData` m
 
 ```ts [server/api/count.ts]
 let counter = 0
-export default () => {
+export default defineEventHandler(() => {
   counter++
   return JSON.stringify(counter)
-}
+})
 ```
 
 ```vue [app.vue]