livecaptions: set mainProgram

Signed-off-by: Sefa Eyeoglu <[e-mail]>

pkgs/applications/misc/livecaptions/default.nix

···
       license = licenses.gpl3Plus;
       platforms = platforms.linux;
       maintainers = with maintainers; [Scrumplex];
+      mainProgram = "livecaptions";
     };
   })