Fixing the file call in the rfc822.Message replacement example.

Doc/includes/email-headers.py

@@ -2,7 +2,7 @@
 from email.parser import Parser
 
 #  If the e-mail headers are in a file, uncomment this line:
-#headers = Parser().parse(messagefile)
+#headers = Parser().parse(open(messagefile, 'r'))
 
 #  Or for parsing headers in a string, use:
 headers = Parser().parsestr('From: <user@example.com>\n'