remove "#" symbols to make the code compile

src/doc/book/closures.md

@@ -492,12 +492,12 @@ fn factory() -> Box<Fn(i32) -> i32> {
 
     Box::new(move |x| x + num)
 }
-# fn main() {
+fn main() {
 let f = factory();
 
 let answer = f(1);
 assert_eq!(6, answer);
-# }
+}
 ```
 
 By making the inner closure a `move Fn`, we create a new stack frame for our