tests: fix update_valid_ee_certs Rust script syntax

The syntax for this unstable cargo feature changed out from under us,
producing syntax errors with recent nightly tooling. This commit updates
the syntax to match the documented[0] example script.

[0]: https://github.com/rust-lang/cargo/blob/master/src/doc/src/reference/unstable.md#script

rustls-platform-verifier/src/tests/verification_real_world/update_valid_ee_certs.rs

@@ -1,11 +1,11 @@
-#!/usr/bin/env -S cargo +nightly -Z script
-```cargo
-package.edition = "2021"
-dependencies.anyhow = "1"
-dependencies.reqwest.version = "0.11"
-dependencies.reqwest.default-features = false
-dependencies.reqwest.features = ["blocking", "rustls-tls-webpki-roots"]
-```
+#!/usr/bin/env -S cargo -Z script
+---cargo
+[package]
+edition = "2021"
+[dependencies]
+anyhow = "1"
+reqwest = { version = "0.11", default-features = false, features = ["blocking", "rustls-tls-webpki-roots"] }
+---
 
 use std::{fs, path::Path};