doc: simplify util.TextDecoder example

This simplifies the example and makes it runnable. (The current example
has a magic function.) (This also removes an assignment in a condition
which will be flagged if we enable ESLint's no-cond-assign rule.)

PR-URL: #41574
Reviewed-By: Mestery <mestery@protonmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

doc/api/util.md

@@ -1185,13 +1185,9 @@ added: v8.3.0
 An implementation of the [WHATWG Encoding Standard][] `TextDecoder` API.
 
 ```js
-const decoder = new TextDecoder('shift_jis');
-let string = '';
-let buffer;
-while (buffer = getNextChunkSomehow()) {
-  string += decoder.decode(buffer, { stream: true });
-}
-string += decoder.decode(); // end-of-stream
+const decoder = new TextDecoder();
+const u8arr = new Uint8Array([72, 101, 108, 108, 111]);
+console.log(decoder.decode(u8arr)); // Hello
 ```
 
 ### WHATWG supported encodings