struct_visibility.md: Remove unneeded '#[allow(dead_code)]' (#1609)

Directive to allow dead code on struct ClosedBox is not needed and confusing.  It is used when calling using the public constructor about line 44: `let _closed_box = my::ClosedBox::new("classified information");`

src/mod/struct_visibility.md

@@ -13,7 +13,6 @@ mod my {
     }
 
     // A public struct with a private field of generic type `T`
-    #[allow(dead_code)]
     pub struct ClosedBox<T> {
         contents: T,
     }
@@ -56,4 +55,4 @@ fn main() {
 [generics][generics] and [methods][methods]
 
 [generics]: ../generics.md
-[methods]: ../fn/methods.md
+[methods]: ../fn/methods.md