MAINTAINERS: update adi,ad5758.yaml reference

Changeset 1e6536ee ("dt-bindings:iio:dac:adi,ad5758 yaml conversion")
renamed: Documentation/devicetree/bindings/iio/dac/ad5758.txt
to: Documentation/devicetree/bindings/iio/dac/adi,ad5758.yaml.

Update its cross-reference accordingly.

Fixes: 1e6536ee ("dt-bindings:iio:dac:adi,ad5758 yaml conversion")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>