demos/gtk-demo/combobox: fix typo

Fix typo that prevented the P-S submenu from displaying correctly

(cherry picked from commit 462193ae)
2 jobs for <span class="ref-name">demo-combobox-typo-gtk3</span> in 9 minutes and 16 seconds (queued for 57 seconds)
Status Job ID Name Coverage
  Build
passed #164605
build

00:09:16

manual #164606
win32 allowed to fail manual
msys2-mingw32