Fix a typo in Chapter 17

Author: marijnh

17_http.txt

@@ -338,7 +338,7 @@ does not know how to handle.
 I/O)))(((responseText property)))(((send method)))In the examples we
 saw, the request has finished when the call to `send` returns. This is
 convenient because it means properties such as `responseText` are
-available immediately. But it also mean that our program is suspended
+available immediately. But it also means that our program is suspended
 as long as the ((browser)) and server are communicating. When the
 ((connection)) is bad, the server is slow, or the file is big, that
 might take quite a while. Worse, because no event handlers can fire