tdm: Fix the build

Was failing in the desktop-file validation stage [0]:
/nix/store/XXX-tdm.desktop/share/applications/tdm.desktop: error: value
"False" for boolean key "Terminal" in group "Desktop Entry" contains
invalid characters, boolean values must be "false" or "true"

[0]: https://hydra.nixos.org/build/123852137

pkgs/games/tdm/default.nix

@@ -11,7 +11,7 @@ let
     name = pname;
     exec = "@out@/bin/${pname}";
     icon = pname;
-    terminal = "False";
+    terminal = "false";
     comment = "The Dark Mod - stealth FPS inspired by the Thief series";
     type = "Application";
     categories = "Game;";