tutorial: fix for-loop example

Although in the example function `each` works as expected with
rust-0.6 (the latest release), it fails to even compile with `incoming`
rust (see test/compile-fail/bad-for-loop-2.rs). Change the function to
return a `bool` instead of `()`: this works fine with both versions of
rust, and does not misguide potential contributors.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>