update deno serve documentation to mention the satisfies type (#1673)

Author: thisisjofrank

runtime/reference/cli/serve.md

@@ -17,9 +17,14 @@ export default {
   async fetch(_req) {
     return new Response("Hello world!");
   },
-};
+} satisfies Deno.ServeDefaultExport;
 ```
 
+The `satisfies Deno.ServeDefaultExport` type assertion ensures that your
+exported object conforms to the expected interface for Deno's HTTP server. This
+provides type safety and better editor autocomplete while allowing you to
+maintain the inferred types of your implementation.
+
 You can then run the server using the `deno serve` command:
 
 ```bash
@@ -38,5 +43,5 @@ export default {
 
     return new Response("Hello world!");
   },
-};
+} satisfies Deno.ServeDefaultExport;
 ```