Actually move out of Box in moving-out-of-Box test

Author: dianne

src/expressions/operator-expr.md

@@ -186,7 +186,7 @@ let y = &mut 9;
 *y = 11;
 assert_eq!(*y, 11);
 let z = Box::new(NoCopy);
-let _: NoCopy = *z;
+let a: NoCopy = *z;
 ```
 
 r[expr.try]