docs: fix the url of router event (#51339)

Refer to [here](https://developer.mozilla.org/en-US/docs/Web/API/URLSearchParams/toString#examples)

The `searchParams.toString()` doesn't include the question mark.


Co-authored-by: Balázs Orbán <18369201+balazsorban44@users.noreply.github.com>

docs/02-app/02-api-reference/04-functions/use-router.mdx

@@ -80,7 +80,7 @@ export function NavigationEvents() {
   const searchParams = useSearchParams()
 
   useEffect(() => {
-    const url = pathname + searchParams.toString()
+    const url = `${pathname}?${searchParams}`
     console.log(url)
     // You can now use the current URL
     // ...