doc: fix invalid link in worker_threads.md

Inadvertent double hash in link makes it invalid and no longer pointing
to the documentation section it intends to point to.

PR-URL: https://github.com/nodejs/node/pull/36109


Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Yongsheng Zhang <zyszys98@gmail.com>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>