doc: extend example of http.request by end event

Added .on('end', callback) event to http.request example, because
for first sight it's not clear from http documentation, how to
handle end of request.

PR-URL: https://github.com/iojs/io.js/pull/447
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

doc/api/http.markdown

@@ -514,6 +514,9 @@ Example:
       res.on('data', function (chunk) {
         console.log('BODY: ' + chunk);
       });
+      res.on('end', function() {
+        console.log('No more data in response.')
+      })
     });
 
     req.on('error', function(e) {