[bug report] erroneous suggestion for "--add" option in `git config set` error message

It seems that there is an erroneous suggestion in a `git config set` error message when setting a value that already has multiple values. The error message says to use the `--add` option, but this option does not seem to exist. The `--append` option should be suggested instead.

Steps to reproduce:

1. Create a configuration with multiple values:

    git config set --global test.test 'hello'
    git config set --global --append test.test 'hi'

2. Try to set the value of `test.test`:

    git config set --global test.test 'hi'

An error message is shown:

    warning: test.test has multiple values
    error: cannot overwrite multiple values with a single value
           Use a regexp, --add or --replace-all to change test.test.

3. Following the direction to use `--add` produces an error:

    > git config set --add test.test "aloha"
    error: unknown option `add'

The error message should say `--append` instead of `--add`.

[System Info] git version: git version 2.48.1 cpu: x86_64 no commit associated with this build sizeof-long: 8 sizeof-size_t: 8 shell-path: /bin/sh libcurl: 8.12.1 zlib: 1.3.1 uname: Linux 6.14.0-35-generic #35-Ubuntu SMP PREEMPT_DYNAMIC Sat Oct 11 10:06:31 UTC 2025 x86_64 compiler info: gnuc: 14.2 libc info: glibc: 2.41 $SHELL (typically, interactive shell): /bin/bash

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-- Paul Wintz, PhD UCSC Hybrid Systems Laboratory Hartnell College, Adjunct Faculty PaulWintz.com