MAINTAINERS: update envelope-detector.yaml reference

MAINTAINERS: update envelope-detector.yaml reference

Changeset 66a6dcc20e63 ("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: 66a6dcc20e63 ("dt-bindings:iio:adc:envelope-detector: txt to yaml conversion.")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/d4ccc625ccb89730c03204b7aae98fd94ea97fc2.1617279355.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>