docs: standardize on serve.nu as example filename

Author: cablehead

README.md

@@ -119,7 +119,7 @@ Hello world
 Or from a file:
 
 ```bash
-$ http-nu :3001 ./handler.nu
+$ http-nu :3001 ./serve.nu
 ```
 
 ### UNIX domain sockets
@@ -135,7 +135,7 @@ Hello world
 Use `-w` / `--watch` to automatically reload when files change:
 
 ```bash
-$ http-nu :3001 -w ./handler.nu
+$ http-nu :3001 -w ./serve.nu
 ```
 
 This watches the script's directory for any changes (including included files)