Removed commented main functions entirely. This fits in with other ex…

…amples in the Closures section

src/doc/book/closures.md

@@ -511,12 +511,10 @@ fn factory() -> Box<Fn(i32) -> i32> {
     Box::new(|x| x + num)
 }
 
-# fn main() {
 let f = factory();
 
 let answer = f(1);
 assert_eq!(6, answer);
-# }
 ```
 
 There’s just one last problem:
@@ -542,12 +540,10 @@ fn factory() -> Box<Fn(i32) -> i32> {
     Box::new(move |x| x + num)
 }
 
-# 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