Fix example to compile

[steveklabnik]

Previously, hoedown did not understand that this was an example. pulldown-cmark does. This fixes the example to be compile-able.

Fixes rust-lang/rust#40994

[steveklabnik]

So, looks like this example overflows? Not sure what specifically you all want to do here.

[alexcrichton]

for ch in 0..255 {
    let ch = ch as u8;
}

maybe?