os::sleep in assert message should be changed to JavaThread::sleep

This assert message still mentions os::sleep.

http://hg.openjdk.java.net/jdk/jdk/file/00aebe177a71/src/hotspot/share/runtime/thread.cpp#l3392

      assert(!os::supports_monotonic_clock(),
             "unexpected time moving backwards detected in os::sleep()");

It should be changed to JavaThread::sleep after JDK-8230423: Move os::sleep to JavaThread::sleep