qemu-doc.texi: Rename references to 2.13 to 3.0

Update references to 2.13 to read 3.0, since that's the
number we're using for the next release.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-id: 20180522104000.9044-6-peter.maydell@linaro.org