stream: fix signature of _write() in a comment

This comment was a bit misleading, since it was missing the `encoding`
argument.

PR-URL: #3248
Reviewed-By: Brendan Ashworth <brendan.ashworth@me.com>

lib/_stream_writable.js

@@ -1,5 +1,5 @@
 // A bit simpler than readable streams.
-// Implement an async ._write(chunk, cb), and it'll handle all
+// Implement an async ._write(chunk, encoding, cb), and it'll handle all
 // the drain event emission and buffering.
 
 'use strict';