MAINTAINERS: update lego,ev3-battery.yaml reference

Changeset 3004e581 ("dt-bindings: power: supply: lego-ev3-battery: Convert to DT schema format")
renamed: Documentation/devicetree/bindings/power/supply/lego_ev3_battery.txt
to: Documentation/devicetree/bindings/power/supply/lego,ev3-battery.yaml.

Update its cross-reference accordingly.

Fixes: 3004e581 ("dt-bindings: power: supply: lego-ev3-battery: Convert to DT schema format")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Reviewed-by: David Lechner <david@lechnology.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>