Fixed small typo Pointed out by Michael Hudson.

Doc/ext/newtypes.tex

@@ -255,7 +255,7 @@ move to that directory and fire up Python --- you should be able to
 That wasn't so hard, was it?
 
 Of course, the current Noddy type is pretty uninteresting. It has no
-data and doesn't do anything. It can't even be subclasses.
+data and doesn't do anything. It can't even be subclassed.
 
 \subsection{Adding data and methods to the Basic example}