Fix up chapter 6
Fix up chapter 6

gpbonillas/rust_for_rubyists,dskecse/rust_for_rubyists,steveklabnik/rust_for_rubyists,dskecse/rust_for_rubyists,steveklabnik/rust_for_rubyists,dskecse/rust_for_rubyists,gpbonillas/rust_for_rubyists,steveklabnik/rust_for_rubyists,gpbonillas/rust_for_rubyists