Fixes a typo introduced at r.9915.

git-svn-id: http://seriss.com/public/fltk/fltk/branches/branch-1.3@9916 ea41ed52-d2ee-0310-a9c1-e6b18d33e121
Fixes Fl_System_Printer::printable_rect() and Fl_System_Printer::margins() that did not work well
after calls to Fl_System_Printer::translate() under MSWindows.

git-svn-id: http://seriss.com/public/fltk/fltk/branches/branch-1.3@9917 ea41ed52-d2ee-0310-a9c1-e6b18d33e121