In commit 6d8980a3 we updated the copyright string we present to
the user in -version output, About dialogs, etc, but we forgot that
the Sphinx manuals have a separate copyright string setting.  Update
that one too.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-id: 20210705095547.15790-2-peter.maydell@linaro.org