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>

Documentation/devicetree/bindings/net/qualcomm-bluetooth.txt

@@ -29,7 +29,7 @@ Required properties for compatible string qcom,wcn399x-bt:
 
 Optional properties for compatible string qcom,wcn399x-bt:
 
- - max-speed: see Documentation/devicetree/bindings/serial/slave-device.txt
+ - max-speed: see Documentation/devicetree/bindings/serial/serial.yaml
  - firmware-name: specify the name of nvm firmware to load
  - clocks: clock provided to the controller