docs(changelog): fix version tag links

The Standard Version tool will not recognize a release header as a
release header without the minor version, and will overwrite them when
generating the next release changelog.

Additionally, it will not generate a link to the tag diff unless a tag
of the form `vX.Y.Z` exists. We ought to generate tags with this version
format in the future to ensure the diff links are generated.

Change-Id: I7864ab7a5822f83ddb7a7917208d2d029ae34729
Signed-off-by: Chris Kay <chris.kay@arm.com>