docs(react-router): update sourcemaps buildEnd hook snippet (#12889)

Author: tbedrnik

platform-includes/getting-started-sourcemaps/javascript.react-router.mdx

@@ -34,10 +34,10 @@ import { sentryOnBuildEnd } from '@sentry/react-router';
 
 export default {
   ssr: true,
-  buildEnd: ({ viteConfig, reactRouterConfig, buildManifest }) => {
+  buildEnd: async ({ viteConfig, reactRouterConfig, buildManifest }) => {
     // ...
     // Call this at the end of the hook
-    sentryOnBuildEnd({ viteConfig, reactRouterConfig, buildManifest });
+    await sentryOnBuildEnd({ viteConfig, reactRouterConfig, buildManifest });
   },
 } satisfies Config;
 ```