doc: adjust assignment in condition in stream doc

This is part of an effort to get our code to comply with ESLint
no-cond-assign so that we don't have to disable that rule in our config.

PR-URL: #41510
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>

doc/api/stream.md

@@ -1002,7 +1002,7 @@ readable.on('readable', function() {
  // There is some data to read now.
  let data;
 
- while (data = this.read()) {
+ while ((data = this.read()) !== null) {
  console.log(data);
  }
 });