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

doc/api/stream.md

@@ -1035,7 +1035,7 @@ 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);
   }
 });