transmute_copy doc example is too trivial

http://doc.rust-lang.org/nightly/std/mem/fn.transmute_copy.html#examples

``` rust
use std::mem;

let one = unsafe { mem::transmute_copy(&1) };

assert_eq!(1, one);
```

This transmutes an `i32` to... an `i32`! A more realistic example might be to transmute a byte array to a packed struct.
