docs: dt: qcom,dwc3.txt: fix cross-reference for a converted file

The qcom-qusb2-phy.txt file was converted and renamed to yaml.
Update cross-reference accordingly.

Fixes: 8ce65d8d ("dt-bindings: phy: qcom,qusb2: Convert QUSB2 phy bindings to yaml")
Reviewed-by: NStephen Boyd <swboyd@chromium.org>
Signed-off-by: NMauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/a055c564f2a79aa748064329d938db8b3c8edd58.1586881715.git.mchehab+huawei@kernel.orgSigned-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/devicetree/bindings/usb/qcom,dwc3.txt

@@ -52,8 +52,8 @@ A child node must exist to represent the core DWC3 IP block. The name of
 the node is not important. The content of the node is defined in dwc3.txt.
 
 Phy documentation is provided in the following places:
-Documentation/devicetree/bindings/phy/qcom-qmp-phy.txt   - USB3 QMP PHY
-Documentation/devicetree/bindings/phy/qcom-qusb2-phy.txt - USB2 QUSB2 PHY
+Documentation/devicetree/bindings/phy/qcom-qmp-phy.txt    - USB3 QMP PHY
+Documentation/devicetree/bindings/phy/qcom,qusb2-phy.yaml - USB2 QUSB2 PHY
 
 Example device nodes: