MAINTAINERS: update lego,ev3-battery.yaml reference

Changeset 3004e581d92a ("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: 3004e581d92a ("dt-bindings: power: supply: lego-ev3-battery: Convert to DT schema format")
Signed-off-by: Mauro Carvalho Chehab <[e-mail]>
Reviewed-by: David Lechner <[e-mail]>
Signed-off-by: Sebastian Reichel <[e-mail]>

MAINTAINERS

···
 LEGO MINDSTORMS EV3
 R:	David Lechner <[e-mail]>
 S:	Maintained
-F:	Documentation/devicetree/bindings/power/supply/lego_ev3_battery.txt
+F:	Documentation/devicetree/bindings/power/supply/lego,ev3-battery.yaml
 F:	arch/arm/boot/dts/da850-lego-ev3.dts
 F:	drivers/power/supply/lego_ev3_battery.c