gnome-frog: set meta.mainProgram

pkgs/applications/misc/gnome-frog/default.nix

···
     description =
       "Intuitive text extraction tool (OCR) for GNOME desktop";
     license = licenses.mit;
+    mainProgram = "frog";
     maintainers = with maintainers; [ foo-dogsquared ];
     platforms = platforms.linux;
   };