Update demo.rs

I've just built rust-nightly and the following error popped out:

```
/src/main.rs:2:1: 2:21 error: can't find crate for `native`
/src/main.rs:2 extern crate native;
                                                ^~~~~~~~~~~~~~~~~~~~
```

So I've removed from your example, ok?
I don't know the real explanation for this, I'm just getting started with Rust.