Updated servo horn attachment

See Trello:

- https://trello.com/c/J61AePhP/69-website-instructions-for-reference-kit-don-t-specify-to-make-sure-wheels-are-screwed-to-the-servos
- https://trello.com/c/zqiBP4Kb/68-website-servos-are-not-attached-to-the-horns-when-out-of-the-bag-so-description-to-detach-is-confusing