docs: dt: fix a broken reference for a file converted to json

docs: dt: fix a broken reference for a file converted to json

Changeset 32ced09d7903 ("dt-bindings: serial: Convert slave-device bindings to json-schema")
moved a binding to json and updated the links.

Yet, one link was not changed, due to a merge conflict.

Update this one too.

Fixes: 32ced09d7903 ("dt-bindings: serial: Convert slave-device bindings to json-schema")
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/9b1603e254d39c9607bfedefeedaafd2c44aeb19.1586881715.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>