Use idiomatic "seq" instead of "not-empty"

[mzuther]

Hi!

To check for empty sequences, it's preferred to use "seq" in Clojure -- see the doc string of (doc empty?). It might seem easier first to use "not-empty", but in that case a novice will not be able to understand idiomatic code. So I think you should teach the "seq" idiom instead of "not-empty".

I also fixed a small typo on the way. :)

Thanks for your work, Clojure by Example is great!

Martin