修复20-11的mimr代码检查反馈。

原数据可能是未安装nightly 版本的 Rust 和 Miri的结果，英文原版网页运行也是这个反馈。

listings/ch20-advanced-features/listing-20-11/output.txt

@@ -1,5 +1,16 @@
 $ cargo +nightly miri run
    Compiling unsafe-example v0.1.0 (file:///projects/unsafe-example)
     Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.01s
-     Running `file:///home/.rustup/toolchains/nightly/bin/cargo-miri runner target/miri/debug/unsafe-example`
+     Running `/Users/chris/.rustup/toolchains/nightly-aarch64-apple-darwin/bin/cargo-miri runner target/miri/aarch64-apple-darwin/debug/unsafe-example`
+warning: creating a shared reference to mutable static is discouraged
+  --> src/main.rs:14:33
+   |
+14 |         println!("COUNTER: {}", COUNTER);
+   |                                 ^^^^^^^ shared reference to mutable static
+   |
+   = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2024/static-mut-references.html>
+   = note: shared references to mutable statics are dangerous; it's undefined behavior if the static is mutated or if a mutable reference is created for it while the shared reference lives
+   = note: `#[warn(static_mut_refs)]` on by default
+
 COUNTER: 3
+