leds: Fix reference to led_set_brightness() in doc

The referenced function led_classdev_brightness_set() never existed.

Fixes: 5ada28bf ("led-class: always implement blinking")
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20230218-typo-led-set-v1-1-3c35362a2f2d@weissschuh.net