docs: replace git_config to repo_config

docs: replace git_config to repo_config

Since this document was written, the built-in API has been
updated a few times, but the document was left stale.

Adjust to the current best practices by calling repo_config() on the
repository instance the subcommand implementation receives as a
parameter, instead of calling git_config() that used to be the
common practice.

Signed-off-by: K Jayatheerth <redacted>
Signed-off-by: Junio C Hamano <redacted>