Documentation: kthread: Fix WQ_SYSFS workqueues path name

The set of WQ_SYSFS workqueues should be displayed using
"ls /sys/devices/virtual/workqueue", add the missing '/'.

Signed-off-by: Zenghui Yu <yuzenghui@huawei.com>
Link: https://lore.kernel.org/r/20200225124052.1506-1-yuzenghui@huawei.com


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