doc: remove superfluous await from fsPromises.readdir example

The `await` operator in the example, iterating over the returned array
of filenames is not necessary, since the returned array is either
consisting of `string`s or of `fs.Dirent` objects, neither providing
an asyncIterator.

Refs: https://github.com/nodejs/help/issues/3317

PR-URL: https://github.com/nodejs/node/pull/38293


Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>