Update SWR example to include fetcher function. (vercel#26520)

* Previous example doesn't work

* Apply suggestions from code review

* Update docs/basic-features/data-fetching.md

* lint-fix

Co-authored-by: Lee Robinson <me@leerob.io>
Co-authored-by: JJ Kasper <jj@jjsweb.site>

docs/basic-features/data-fetching.md

@@ -825,8 +825,10 @@ The team behind Next.js has created a React hook for data fetching called [**SWR
 ```jsx
 import useSWR from 'swr'
 
+const fetcher = (url) => fetch(url).then((res) => res.json())
+
 function Profile() {
-  const { data, error } = useSWR('/api/user', fetch)
+  const { data, error } = useSWR('/api/user', fetcher)
 
   if (error) return <div>failed to load</div>
   if (!data) return <div>loading...</div>