Update chapter-06.md
Update chapter-06.md
steveklabnik/rust_for_rubyists,gpbonillas/rust_for_rubyists,gpbonillas/rust_for_rubyists,gpbonillas/rust_for_rubyists,dskecse/rust_for_rubyists,dskecse/rust_for_rubyists,steveklabnik/rust_for_rubyists,dskecse/rust_for_rubyists,steveklabnik/rust_for_rubyists