docs: fix command examples use outdated default file_server port (den…

…oland/deno#8062)

http/README.md

@@ -15,7 +15,7 @@ A small program for serving local files over HTTP.
 
 ```sh
 deno run --allow-net --allow-read https://deno.land/std/http/file_server.ts
-> HTTP server listening on http://0.0.0.0:4500/
+> HTTP server listening on http://0.0.0.0:4507/
 ```
 
 ## Cookie