Dedup the Rust 2018 module system quote

[dwijnand]

Both https://rust-lang-nursery.github.io/edition-guide/2018/transitioning/modules/index.html and https://rust-lang-nursery.github.io/edition-guide/2018/transitioning/modules/path-clarity.html contain the quote that starts with "while there are simple and consistent rules...".

This is particularly obvious if consuming the single-page format https://rust-lang-nursery.github.io/edition-guide/print.html.

See #15 for a suggested change that rephrase the intro without the quote.