backlight: backlight: Fix doc for backlight_device_get_by_name

backlight_put() has been dropped, we should call put_device() to drop
the reference taken by backlight_device_get_by_name().

Fixes: 0f6a3256 ("backlight: backlight: Drop backlight_put()")
Signed-off-by: Miaoqian Lin <linmq006@gmail.com>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20221215071902.424005-1-linmq006@gmail.com