update lazy_static! to not use let

Author: AndyGauge

src/mem/global_static/lazy-constant.md

@@ -40,14 +40,16 @@ fn main() {
 ```rust,edition2021
 use std::cell::OnceCell;
 
-let cell = OnceCell::new();
-assert!(cell.get().is_none());
-
-let value: &String = cell.get_or_init(|| {
-    "Hello, World!".to_string()
-});
-assert_eq!(value, "Hello, World!");
-assert!(cell.get().is_some());
+fn main() {
+    let cell = OnceCell::new();
+    assert!(cell.get().is_none());
+
+    let value: &String = cell.get_or_init(|| {
+        "Hello, World!".to_string()
+    });
+    assert_eq!(value, "Hello, World!");
+    assert!(cell.get().is_some());
+}
 ```
 
 [`OnceCell`]: https://doc.rust-lang.org/beta/std/cell/struct.OnceCell.html