Fix typos in library/email.policy.rst

Fixed `Defect` to `MessageDefect` since the class `Defect` does not exist.

Author: Yuki Kobayashi

Doc/library/email.policy.rst

@@ -267,7 +267,7 @@ added matters.  To illustrate::
 
       Handle a *defect* found on *obj*.  When the email package calls this
       method, *defect* will always be a subclass of
-      :class:`~email.errors.Defect`.
+      :class:`~email.errors.MessageDefect`.
 
       The default implementation checks the :attr:`raise_on_defect` flag.  If
       it is ``True``, *defect* is raised as an exception.  If it is ``False``
@@ -277,7 +277,7 @@ added matters.  To illustrate::
    .. method:: register_defect(obj, defect)
 
       Register a *defect* on *obj*.  In the email package, *defect* will always
-      be a subclass of :class:`~email.errors.Defect`.
+      be a subclass of :class:`~email.errors.MessageDefect`.
 
       The default implementation calls the ``append`` method of the ``defects``
       attribute of *obj*.  When the email package calls :attr:`handle_defect`,