builtin/reflog.c: fix dry-run option short name

builtin/reflog.c: fix dry-run option short name

The documentation for reflog states that the --dry-run option of the
expire and delete subcommands has a corresponding short name, -n.
However, 33d7bdd645 (builtin/reflog.c: use parse-options api for expire,
delete subcommands, 2022-01-06) did not include this short name in the
new options parsing.

Re-add the short name in the new dry-run option definitions.

Signed-off-by: Josh Brobst <josh@brob.st>
Signed-off-by: Junio C Hamano <gitster@pobox.com>