doc: add missing function call to example for util.promisify

PR-URL: nodejs#49719
Reviewed-By: Moshe Atlow <moshe@atlow.co.il>
Reviewed-By: Deokjin Kim <deokjin81.kim@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

doc/api/util.md

@@ -1635,6 +1635,8 @@ async function callStat() {
   const stats = await stat('.');
   console.log(`This directory is owned by ${stats.uid}`);
 }
+
+callStat();
 ```
 
 If there is an `original[util.promisify.custom]` property present, `promisify`