MAINTAINERS: update envelope-detector.yaml reference

Changeset 66a6dcc2 ("dt-bindings:iio:adc:envelope-detector: txt to yaml conversion.")
renamed: Documentation/devicetree/bindings/iio/adc/envelope-detector.txt
to: Documentation/devicetree/bindings/iio/adc/envelope-detector.yaml.

Update its cross-reference accordingly.

Fixes: 66a6dcc2 ("dt-bindings:iio:adc:envelope-detector: txt to yaml conversion.")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>