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

Changeset 32ced09d ("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: 32ced09d ("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>
Signed-off-by: Rob Herring <robh@kernel.org>