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: Stephen Boyd <swboyd@chromium.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/a055c564f2a79aa748064329d938db8b3c8edd58.1586881715.git.mchehab+huawei@kernel.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>