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>

MAINTAINERS

@@ -14287,7 +14287,7 @@ PNI RM3100 IIO DRIVER
 M:	Song Qiang <songqiang1304521@gmail.com>
 L:	linux-iio@vger.kernel.org
 S:	Maintained
-F:	Documentation/devicetree/bindings/iio/magnetometer/pni,rm3100.txt
+F:	Documentation/devicetree/bindings/iio/magnetometer/pni,rm3100.yaml
 F:	drivers/iio/magnetometer/rm3100*
 
 PNP SUPPORT