unrar-free: fix meta.mainProgram (NixOS#339905)

Author: thiagokokada

pkgs/by-name/un/unrar-free/package.nix

@@ -34,7 +34,7 @@ stdenv.mkDerivation (finalAttrs: {
     '';
     homepage = "https://gitlab.com/bgermann/unrar-free";
     license = lib.licenses.gpl2Plus;
-    mainProgram = "unrar";
+    mainProgram = "unrar-free";
     maintainers = with lib.maintainers; [ thiagokokada ];
     platforms = lib.platforms.unix;
     broken = stdenv.isDarwin;