Copy code funtion always includes fn main

Hi,

when I use the copy code button on the code snippets in the tutorial it always includes an fn main.
For example [Chapter 3](http://bfnightly.bracketproductions.com/rustbook/chapter_3.html) first snippet when I copy I get:

```

#![allow(unused_variables)]
fn main() {
/// Makes a map with solid boundaries and 400 randomly placed walls. No guarantees that it won't
/// look awful.
pub fn new_map_test() -> Vec {
...
}
}
```