Add ignore to doc code

Author: qnighy

src/doc/unstable-book/src/library-features/fnbox.md

@@ -8,15 +8,15 @@ The tracking issue for this feature is [#28796]
 
 This had been a temporary alternative to the following impls:
 
-```rust
+```rust,ignore
 impl<A, F> FnOnce for Box<F> where F: FnOnce<A> + ?Sized {}
 impl<A, F> FnMut for Box<F> where F: FnMut<A> + ?Sized {}
 impl<A, F> Fn for Box<F> where F: Fn<A> + ?Sized {}
 ```
 
 The impls are parallel to these (relatively old) impls:
 
-```rust
+```rust,ignore
 impl<A, F> FnOnce for &mut F where F: FnMut<A> + ?Sized {}
 impl<A, F> FnMut for &mut F where F: FnMut<A> + ?Sized {}
 impl<A, F> Fn for &mut F where F: Fn<A> + ?Sized {}