doc: fix comma of the list in worker_threads.md
Checklist
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make -j4 test(UNIX), orvcbuild test(Windows) passes -
tests and/or benchmarks are included -
documentation is changed or added -
commit message follows commit guidelines
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
make -j4 test (UNIX), or vcbuild test (Windows) passes