docs: dt: fix a broken reference to input.yaml

The old file was converted to yaml, but its reference was
still pointing to the old one.

Fixes: 7cef1079 ("dt-bindings: input: Add common input binding in json-schema")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>