docs/interop/firmware: Literal block markup

docs/interop/firmware: Literal block markup

A few doc comments show command line snippets.  The snippets are
indented, which is legible enough.  Actually formatting these with
Sphinx would fail with "Unexpected indentation", though.  We don't so
far.  Add suitable markup anyway.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Message-ID: <20251103082354.3273027-9-armbru@redhat.com>