cargo script example needs nightly -Zscript feature

src/doc/src/reference/unstable.md

@@ -1407,7 +1407,7 @@ fn main() {}
 
 A user may optionally specify a manifest in a `cargo` code fence in a module-level comment, like:
 ```rust
-#!/usr/bin/env cargo
+#!/usr/bin/env cargo +nightly -Zscript
 
 //! ```cargo
 //! [dependencies]