docs/devel: Update URL for make-pullreq script

In the submitting-a-pull-request docs, we have a link to the
make-pullreq script which might be useful for maintainers.  The
canonical git repo for this script has moved; update the link.

Cc: qemu-stable@nongnu.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-id: 20251125164511.255550-1-peter.maydell@linaro.org