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>

MAINTAINERS

@@ -8682,7 +8682,7 @@ M:	Peter Rosin <peda@axentia.se>
 L:	linux-iio@vger.kernel.org
 S:	Maintained
 F:	Documentation/ABI/testing/sysfs-bus-iio-adc-envelope-detector
-F:	Documentation/devicetree/bindings/iio/adc/envelope-detector.txt
+F:	Documentation/devicetree/bindings/iio/adc/envelope-detector.yaml
 F:	drivers/iio/adc/envelope-detector.c
 
 IIO MULTIPLEXER