dt-bindings: leds: Fix JSON pointer in max-brightness

A valid JSON pointer should begin with a '/'. The json-schema package is
lax on this allowing either form, but that's changing in new versions.

Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20231122205418.2482076-1-robh@kernel.org
Signed-off-by: Lee Jones <lee@kernel.org>