walk(): Fix docstring; traversal is depth-first. Closes mimelib bug

#477864.

Lib/email/Message.py

@@ -423,7 +423,7 @@ class Message:
     def walk(self):
         """Walk over the message tree, yielding each subpart.
 
-        The walk is performed in breadth-first order.  This method is a
+        The walk is performed in depth-first order.  This method is a
         generator.
         """
         yield self