docs(server): fix satisfies Serve type in export default example (#25410)

Author: robobun

docs/runtime/http/server.mdx

@@ -193,15 +193,17 @@ This is the maximum amount of time a connection is allowed to be idle before the
 Thus far, the examples on this page have used the explicit `Bun.serve` API. Bun also supports an alternate syntax.
 
 ```ts server.ts
-import { type Serve } from "bun";
+import type { Serve } from "bun";
 
 export default {
   fetch(req) {
     return new Response("Bun!");
   },
-} satisfies Serve;
+} satisfies Serve.Options<undefined>;
 ```
 
+The type parameter `<undefined>` represents WebSocket data — if you add a `websocket` handler with custom data attached via `server.upgrade(req, { data: ... })`, replace `undefined` with your data type.
+
 Instead of passing the server options into `Bun.serve`, `export default` it. This file can be executed as-is; when Bun sees a file with a `default` export containing a `fetch` handler, it passes it into `Bun.serve` under the hood.
 
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