workqueue: fix a kernel-doc warning

As warned by Sphinx:

	./Documentation/core-api/workqueue:400: ./kernel/workqueue.c:1218: WARNING: Unexpected indentation.

the return code table is currently not recognized, as it lacks
markups.

Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>