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: https://github.com/nodejs/node/pull/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>