doc: Fix heading levels for changelog

The change-log.rst file does not use the same symbols
as the other documents when organising its headings,
sub-headings and so on. In order to compile these
documents together with Sphinx, forming a top-level
index and a consistent structure, it is necessary to
normalise these.

Change-Id: Ib4620ff03a9e76fec9e36e95549016c7b3fe12bb
Signed-off-by: Paul Beesley <paul.beesley@arm.com>