Revert ref usage tweak

Author: markerikson

docs/usage/nextjs.mdx

@@ -181,7 +181,7 @@ export default function StoreProvider({
   children: React.ReactNode
 }) {
   // highlight-start
-  const storeRef = useRef<AppStore>(undefined)
+  const storeRef = useRef<AppStore>()
   if (!storeRef.current) {
     // Create the store instance the first time this renders
     storeRef.current = makeStore()