doc: remove _writableState reference

PR-URL: #20004
Fixes: #6799
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/stream.md

@@ -1652,8 +1652,7 @@ const { StringDecoder } = require('string_decoder');
 class StringWritable extends Writable {
   constructor(options) {
     super(options);
-    const state = this._writableState;
-    this._decoder = new StringDecoder(state.defaultEncoding);
+    this._decoder = new StringDecoder(options && options.defaultEncoding);
     this.data = '';
   }
   _write(chunk, encoding, callback) {