Pass reference pointer not box pointer
Pass reference pointer not box pointer

- This is one of many ways to fix this problem. Another would be
  to have plus_one accept a Box<int>, but that would require altering
  the text accompanying the code and is more than plus_one needs.

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