doc: fix sync comment in observer snippet

PR-URL: nodejs#41262
Refs: nodejs#41259
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Mohammed Keyvanzadeh <mohammadkeyvanzade94@gmail.com>
Reviewed-By: Harshitha K P <harshitha014@gmail.com>

doc/api/perf_hooks.md

@@ -647,7 +647,7 @@ const {
 } = require('perf_hooks');
 
 const obs = new PerformanceObserver((list, observer) => {
-  // Called three times synchronously. `list` contains one item.
+  // Called once asynchronously. `list` contains three items.
 });
 obs.observe({ type: 'mark' });