dt-bindings: net: btusb: change reference file name

Due to usb-device.txt is converted into usb-device.yaml,
so modify reference file names at the same time.
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Link: https://lore.kernel.org/r/20201225075258.33352-2-chunfeng.yun@mediatek.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/net/btusb.txt

@@ -4,7 +4,7 @@ Generic Bluetooth controller over USB (btusb driver)
 Required properties:
 
   - compatible : should comply with the format "usbVID,PID" specified in
-		 Documentation/devicetree/bindings/usb/usb-device.txt
+		 Documentation/devicetree/bindings/usb/usb-device.yaml
 		 At the time of writing, the only OF supported devices
 		 (more may be added later) are: