MAINTAINERS: update pni,rm3100.yaml reference

Changeset f383069b ("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: f383069b ("dt-bindings:iio:magnetometer:pni,rm3100: txt to yaml conversion.")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>