Add missing argument to libusb_wait_for_event() documentation, fix #55

Add missing argument to libusb_wait_for_event() documentation, fix #55

In the libusb_wait_for_event() sample code in the section "Letting other
threads do the work for you" the call to libusb_wait_for_event() was not
updated by commit 1df713d622ab4f0b03aad72d903ac7beb8fb3b90, which added
the libusb_context *ctx parameter to the function, so the sample code
was broken until now.