dt-bindings: leds: common: fix example for gpio-leds

The preferred form for gpio-leds compatible subnodes is:
^led-[0-9a-f]$
Fix example by changing led0 and led1 to led-0 and led-1.

Signed-off-by: Johan Jonker <jbx6244@gmail.com>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Rob Herring <robh@kernel.org>