doc: fix typo in worker_threads.md

"Buffer.alloc()" -> "Buffer.allocUnsafe()"

Buffer.alloc() does not use the internal Buffer pool.

PR-URL: https://github.com/nodejs/node/pull/38368


Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>