MAINTAINERS: update st,lsm6dsx.yaml reference

Changeset 7a2cf8e9 ("dt-bindings:iio:imu:st,lsm6dsx: txt to yaml conversion")
renamed: Documentation/devicetree/bindings/iio/imu/st_lsm6dsx.txt
to: Documentation/devicetree/bindings/iio/imu/st,lsm6dsx.yaml.

Update its cross-reference accordingly.

Fixes: 7a2cf8e9 ("dt-bindings:iio:imu:st,lsm6dsx: txt to yaml conversion")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>