Fix docs for unstable script feature

Author: justsmth

src/doc/src/reference/unstable.md

@@ -1239,10 +1239,10 @@ fn main() {}
 A user may optionally specify a manifest in a `cargo` code fence in a module-level comment, like:
 ````rust
 #!/usr/bin/env -S cargo +nightly -Zscript
-```cargo
+---cargo
 [dependencies]
 clap = { version = "4.2", features = ["derive"] }
-```
+---
 
 use clap::Parser;