docs: fix new experimental serialized config option name (#10842)

src/content/docs/en/reference/experimental-flags/serialized-configuration.mdx

@@ -19,13 +19,13 @@ This feature allows access to the `astro:config` virtual module which exposes a
 
 For a complete overview, and to give feedback on this experimental API, see the [Serialized Manifest RFC](https://github.com/withastro/roadmap/blob/feat/serialised-config/proposals/0051-serialized-manifest.md).
 
-To enable this virtual module, add the `experimental.serializeManifest` feature flag to your Astro config:
+To enable this virtual module, add the `experimental.serializeConfig` feature flag to your Astro config:
 
 ```js title="astro.config.mjs" ins={3-5}
 import {defineConfig} from "astro/config"
 export default defineConfig({
   experimental: {
-    serializeManifest: true
+    serializeConfig: true
   }
 })
 ```