stremio: fix meta.mainProgram

Signed-off-by: lucasew <[e-mail]>

pkgs/applications/video/stremio/default.nix

···
   '';
 
   meta = with lib; {
+    mainProgram = "stremio";
     description = "A modern media center that gives you the freedom to watch everything you want.";
     homepage = "https://www.stremio.com/";
     # (Server-side) web UI is closed source now, apparently they work on open-sourcing it.