Fixed formatting for PEP 549. ("Content-type".)

The "Content-type" header was missing.  Apparently
the ".rst" extension is insufficient.

pep-0549.rst

@@ -3,8 +3,10 @@ Title: Instance Properties
 Version: $Revision$
 Last-Modified: $Date$
 Author: larry@hastings.org (Larry Hastings)
+Discussions-To: Python-Dev <python-dev@python.org>
 Status: Draft
 Type: Standards Track
+Content-Type: text/x-rst
 Created: 4-Sep-2017
 Python-Version: 3.7
 Post-History: 4-Sep-2017
@@ -110,9 +112,11 @@ Copyright
 This document has been placed in the public domain.
 
 
-
-Local Variables:
-mode: indented-text
-indent-tabs-mode: nil
-fill-column: 70
-End:
+..
+   Local Variables:
+   mode: indented-text
+   indent-tabs-mode: nil
+   sentence-end-double-space: t
+   fill-column: 70
+   coding: utf-8
+   End: