MyFirstContribution: use struct repository in examples

MyFirstContribution: use struct repository in examples

Add the parameter `struct repository *repo` to the cmd_walken function.

Since commit 9b1cb5070f (builtin: add a repository parameter for
builtin functions, 2024-09-13), all the cmd_* have the `repo` parameter
and new commands must follow this convention, so the documentation
should also be changed.

Change the `git_config` calls to `repo_config`, also passing the `repo`
parameter, as since 036876a106 (config: hide functions using
`the_repository` by default, 2024-08-13) the non-repo config functions
are no longer recommended as they use the global `repository` variable.

Helped-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Lucas Seiki Oshiro <lucasseikioshiro@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>