Replace deleted README demo script reference (#8492)

* Initial plan

* Fix stale README example

Assisted-by: Copilot: GPT-5.6

Co-authored-by: youknowone <69878+youknowone@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: youknowone <69878+youknowone@users.noreply.github.com>

README.md

@@ -41,7 +41,7 @@ needed to prevent stack overflow on Windows):
 
 ```bash
 $ cd RustPython
-$ cargo run --release demo_closures.py
+$ cargo run --release -- -c 'print("Hello, RustPython!")'
 Hello, RustPython!
 ```