doc: pass string to textEncoder.encode as input

Input of `textEncoder.encode` should be string, not array.

Refs: https://nodejs.org/dist/latest-v19.x/docs/api/util.html#textencoderencodeinput
PR-URL: #46421
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

Author: deokjinkim

doc/api/webstreams.md

@@ -1480,7 +1480,7 @@ const { Readable } = require('node:stream');
 const { TextEncoder } = require('node:util');
 
 const encoder = new TextEncoder();
-const dataArray = encoder.encode(['hello world from consumers!']);
+const dataArray = encoder.encode('hello world from consumers!');
 const readable = Readable.from(dataArray);
 arrayBuffer(readable).then((data) => {
   console.log(`from readable: ${data.byteLength}`);