Guide: Make add_three_times_four() public

[cakebaker]

To avoid a compilation error when running the tests the function has to be public.

[steveklabnik]

Thanks!

[steveklabnik]

Unsure what to do here. I guess this must be marked ignore? 😦

[huonw]

It should work if you add a hidden # fn main() {} to override rustdoc's automatic insertion, i.e.

```rust
# fn main() {}
pub fn add_three_times_four(...) { ... }
```

[steveklabnik]

ah ha! I knew there would be a way.

[steveklabnik]

@cakebaker would you mind fixing it in the way @huonw suggested? r=me when that's done.

[cakebaker]

@steveklabnik done

[steveklabnik]

Thank you! Travis failures seem unrelated...