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 <anna@addaleax.net>
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Zijian Liu <lxxyxzj@gmail.com>

Author: RA80533

doc/api/fs.md

@@ -402,7 +402,7 @@ try {
   filehandle = await open('temp.txt', 'r+');
   await filehandle.truncate(4);
 } finally {
-  filehandle?.close();
+  await filehandle?.close();
 }
 ```