doc: remove redundant parameter in `end` listener.

PR-URL: https://github.com/iojs/io.js/pull/1387
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>

doc/api/stream.markdown

@@ -284,7 +284,7 @@ data.
 ```javascript
 var readable = getReadableStreamSomehow();
 readable.resume();
-readable.on('end', function(chunk) {
+readable.on('end', function() {
   console.log('got to the end, but did not read anything');
 });
 ```