MAINTAINERS: update st,hts221.yaml reference

Changeset 9a6ac313 ("dt-bindings:iio:humidity:st,hts221 yaml conversion.")
renamed: Documentation/devicetree/bindings/iio/humidity/hts221.txt
to: Documentation/devicetree/bindings/iio/humidity/st,hts221.yaml.

Update its cross-reference accordingly.

Fixes: 9a6ac313 ("dt-bindings:iio:humidity:st,hts221 yaml conversion.")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>