fix "Reverse Mode" example for Julia 1.11 and 1.12

[matthias314]

This fixes the "Reverse Mode" example from the README file (mentioned in #46) for Julia 1.11 and 1.12. For 1.13.0-rc1 there is another issue:

julia> initrepl(Meta.parse,
                show_function = backwards_show,
                prompt_text = "reverse_julia> ",
                start_key = ')',
                mode_name = "reverse mode")
ERROR: UndefVarError: `setup_search_keymap` not defined in `REPL.LineEdit`

However, this also affects the other examples that work fine in 1.11 and 1.12.