fix custom server example

Author: ScriptedAlchemy

examples/custom-node-server/server/app.ts

@@ -7,7 +7,7 @@ declare module 'react-router' {
   }
 }
 
-export const index = createRequestHandler({
+export const app = createRequestHandler({
   // @ts-expect-error - virtual module provided by React Router at build time
   build: () => import('virtual/react-router/server-build'),
   getLoadContext() {