"Since X.Y" is not recognized as a tagged section, and therefore not
formatted as such in generated documentation.  Fix by adding the
required colon.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20220422132807.1704411-1-armbru@redhat.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>