leds: Fix reference file name of documentation

In commit 56b01acc ("dt-bindings: gpio: fairchild,74hc595:
Convert to json-schema"), gpio-74x164.txt was deleted and replaced
by fairchild,74hc595.yaml. Fix the reference file name.

Signed-off-by: Wan Jiabing <wanjiabing@vivo.com>
Acked-by: Pavel Machek <pavel@ucw.cz>
Link: https://lore.kernel.org/r/20210506070824.10965-1-wanjiabing@vivo.com


Signed-off-by: Rob Herring <robh@kernel.org>