Merge pull request #1106 from ajdavis/patch-2

Bugfix in asynchronous_fetch example code.

docs/guide/async.rst

@@ -78,7 +78,7 @@ callback argument::
         http_client = AsyncHTTPClient()
         def handle_response(response):
             callback(response.body)
-        http_client.fetch(url)
+        http_client.fetch(url, callback=handle_response)
 
 And again with a `.Future` instead of a callback::