doc: interpret-trailers: don’t use deprecated config
`command` has been deprecated since commit c364b7ef51 (trailer: add new .cmd config option, 2021-05-03). Use the commit message of c364b7ef51 as a guide to replace the use of `$ARG` and to use a script instead of an inline command.[1] Also, explicitly trigger the command by passing in `--trailer=see`, since this config is not automatically used.[2] [1]: “Instead of "$ARG", users can refer to the value as positional argument, $1, in their scripts.” [2]: “At the same time, in order to allow `git interpret-trailers` to better simulate the behavior of `git command -s`, 'trailer.<token>.cmd' will not automatically execute.” Acked-by: ZheNing Hu <adlternative@gmail.com> Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name> Signed-off-by: Junio C Hamano <gitster@pobox.com>