Do not call "knee" a standard module, and point to the new location.

This addresses the issue in SF bug #515745.

Doc/lib/libimp.tex

@@ -255,6 +255,6 @@ def __import__(name, globals=None, locals=None, fromlist=None):
 
 A more complete example that implements hierarchical module names and
 includes a \function{reload()}\bifuncindex{reload} function can be
-found in the standard module \module{knee}\refstmodindex{knee} (which
-is intended as an example only --- don't rely on any part of it being
-a standard interface).
+found in the module \module{knee}\refmodindex{knee}.  The
+\module{knee} module can be found in \file{Demo/imputil/} in the
+Python source distribution.