fixed typo as suggested by Hello71 in http://nodebeginner.org/#comment-187982232

Author: manuelkiessling

index.html

@@ -798,7 +798,7 @@ <h4>Event-driven callbacks</h4>
 				callbacks in action :-)
 			</p>
 			<p>
-				(Note that our server will probably write "Requested received."
+				(Note that our server will probably write "Request received."
 				to STDOUT two times upon opening the page in a browser. That's
 				because most browser will try to load the favicon by requesting
 				http://localhost:8888/favicon.ico whenever you open