doc: minor edit to worker_threads.md

Change "JS code" to "JavaScript".

PR-URL: https://github.com/nodejs/node/pull/26870


Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>