Fix --num-tasks description

[HiroakiMikami]

The description of --num-tasks seems to be wrong, and this PR fixes the description.

--num-tasks description says that "if the value is 0, worker will process infinite number of tasks".
But when I set --num-tasks to 0, the worker exits soon after initialization.

related code:

pftaskqueue/pkg/worker/worker.go

Line 128 in 5599568

for i := w.config.NumTasks; i != 0; i-- {

[everpeace]

hmm... should we fix in the case of 0 so as to process tasks infinitely??