doc: fix sync comment in observer snippet

PR-URL: nodejs#41262
Refs: nodejs#41259
Reviewed-By: Luigi Pinca <[email protected]>
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Mohammed Keyvanzadeh <[email protected]>
Reviewed-By: Harshitha K P <[email protected]>

Author: erj826

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' });