workqueue: Fix htmldocs build warning

Fix htmldocs build warning introduced by ec0a7d44 ("workqueue: Add
interface for user-defined workqueue lockdep map").

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Matthew Brost <matthew.brost@intel.com>