doc: use await in filehandle.truncate() snippet

The example snippet of filehandle.close() uses the `await` keyword
based on convention. This change updates the example snippet of
filehandle.truncate() to similarly use the keyword for the purposes of
consistency.

PR-URL: #38939
Reviewed-By: Anna Henningsen <[email protected]>
Reviewed-By: Darshan Sen <[email protected]>
Reviewed-By: Luigi Pinca <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: Zijian Liu <[email protected]>

Author: RA80533

doc/api/fs.md

@@ -394,7 +394,7 @@ try {
   filehandle = await open('temp.txt', 'r+');
   await filehandle.truncate(4);
 } finally {
-  filehandle?.close();
+  await filehandle?.close();
 }
 ```