Documentation/driver-api/usb: Replace dead link

This link is dead:

    $ curl -vI http://usb.cs.tum.edu/usbdoc
    * Could not resolve host: usb.cs.tum.edu
    * Closing connection 0
    curl: (6) Could not resolve host: usb.cs.tum.edu

I found the document somewhere else. Let's replace it.
Signed-off-by: Stefan Tatschner <stefan.tatschner@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/driver-api/usb/writing_usb_driver.rst

@@ -321,6 +321,6 @@ linux-usb-devel Mailing List Archives:
 http://marc.theaimsgroup.com/?l=linux-usb-devel
 
 Programming Guide for Linux USB Device Drivers:
-http://usb.cs.tum.edu/usbdoc
+http://lmu.web.psi.ch/docu/manuals/software_manuals/linux_sl/usb_linux_programming_guide.pdf
 
 USB Home Page: http://www.usb.org