Update class docstring to reflect IOP framework for message persistence

src/iop/_message.py

@@ -1,6 +1,6 @@
 class _Message:
     """ The abstract class that is the superclass for persistent messages sent from one component to another.
     This class has no properties or methods. Users subclass Message and add properties.
-    The PEX framework provides the persistence to objects derived from the Message class.
+    The IOP framework provides the persistence to objects derived from the Message class.
     """
     pass