doc: "finish" event is on the writable stream

doc/api/stream.markdown

@@ -537,7 +537,7 @@ for (var i = 0; i < 100; i ++) {
   writer.write('hello, #' + i + '!\n');
 }
 writer.end('this is the end\n');
-write.on('finish', function() {
+writer.on('finish', function() {
   console.error('all writes are now complete.');
 });
 ```