MAINTAINERS: update pni,rm3100.yaml reference

MAINTAINERS: update pni,rm3100.yaml reference

Changeset f383069be33e ("dt-bindings:iio:magnetometer:pni,rm3100: txt to yaml conversion.")
renamed: Documentation/devicetree/bindings/iio/magnetometer/pni,rm3100.txt
to: Documentation/devicetree/bindings/iio/magnetometer/pni,rm3100.yaml.

Update its cross-reference accordingly.

Fixes: f383069be33e ("dt-bindings:iio:magnetometer:pni,rm3100: txt to yaml conversion.")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/d9090dc18907b4c534bf12a47e47a96ed1d3b45a.1617279355.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>