doc: log: fix --decorate description list

doc: log: fix --decorate description list

026f2e3b (doc: convert git-log to new documentation format, 2025-07-07)
transformed the inline description of `--decorate` options to a
description list:

    We also transform inline descriptions of possible values of option
    --decorate into a list, which is more readable and extensible.

But a source code block was used instead of an open block.

Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>