docs: fix heading styles in upgrading page

The CentOS Stream 8 section used the wrong heading style, which broke
the rest of the document, making the other headings the wrong sizes.

Change-Id: I8015c905074016c53c803ce40a299ad5c911ada6

doc/source/upgrading.rst

@@ -7,7 +7,7 @@ Upgrading
 This section describes how to upgrade from one OpenStack release to another.
 
 CentOS Stream 8
----------------
+===============
 
 The Wallaby release adds support for CentOS Stream 8 as a host operating
 system. CentOS Stream 8 support will also be added to a Victoria stable