doc: add explicit declaration of fd with null val

In example of implementing a writable stream with extending on
Writable add explicit declaration of 'fd' (file descriptor)
variable with null value.
It will make this example more similar to readable stream's one.
And will make it easier to figure out in topic.

PR-URL: nodejs#40704
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Minwoo Jung <nodecorelab@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>

doc/api/stream.md

@@ -2612,6 +2612,7 @@ class WriteStream extends Writable {
   constructor(filename) {
     super();
     this.filename = filename;
+    this.fd = null;
   }
   _construct(callback) {
     fs.open(this.filename, (err, fd) => {