doc: add write flag when open file as the demo code's intention

PR-URL: #54626 Reviewed-By: James M Snell <[email protected]> Reviewed-By: Jake Yuesong Li <[email protected]>

Author: robberfree

doc/api/stream.md

@@ -3731,7 +3731,7 @@ class WriteStream extends Writable{
 this.fd=null;
  }
 _construct(callback){
-fs.open(this.filename, (err, fd) =>{
+fs.open(this.filename, 'w', (err, fd) =>{
 if (err){
 callback(err);
  } else{