SwDoc::CopyPageDesc(): probably sending Modify for first too cannot hurt

Change-Id: I2056bee7555d3f6723ac374863187d82e4ad9edd
(cherry picked from commit 2e9840d18c09ce5552eec2f9d489d952da1034ae)
Reviewed-on: https://gerrit.libreoffice.org/10775
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>