doc: show options arg to new Worker is optional

PR-URL: https://github.com/nodejs/node/pull/21508
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>

doc/api/worker_threads.md

@@ -304,7 +304,7 @@ if (isMainThread) {
 }
 ```
 
-### new Worker(filename, options)
+### new Worker(filename[, options])
 
 * `filename` {string} The absolute path to the Worker’s main script.
   If `options.eval` is true, this is a string containing JavaScript code rather