dt-bindings: power: update battery.yaml reference

Changesets: 70c23e62 ("dt-bindings: power: supply: Fix remaining battery.txt links")
and: 471dec80 ("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: 70c23e62 ("dt-bindings: power: supply: Fix remaining battery.txt links")
Fixes: 471dec80

 ("dt-bindings: power: Convert battery.txt to battery.yaml")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>