dt-bindings: power: update battery.yaml reference

Changesets: 70c23e62d20c ("dt-bindings: power: supply: Fix remaining battery.txt links")
and: 471dec8023d1 ("dt-bindings: power: Convert battery.txt to battery.yaml")
renamed: Documentation/devicetree/bindings/power/supply/battery.txt
to: Documentation/devicetree/bindings/power/supply/battery.yaml.

Update its cross-reference accordingly.

Fixes: 70c23e62d20c ("dt-bindings: power: supply: Fix remaining battery.txt links")
Fixes: 471dec8023d1 ("dt-bindings: power: Convert battery.txt to battery.yaml")
Signed-off-by: Mauro Carvalho Chehab <[e-mail]>
Signed-off-by: Sebastian Reichel <[e-mail]>

Documentation/power/power_supply_class.rst

···
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 Drivers should call power_supply_get_battery_info() to obtain battery
 characteristics from a devicetree battery node, defined in
-Documentation/devicetree/bindings/power/supply/battery.txt. This is
+Documentation/devicetree/bindings/power/supply/battery.yaml. This is
 implemented in drivers/power/supply/bq27xxx_battery.c.
 
 Properties in struct power_supply_battery_info and their counterparts in the