Set terrain arg to SetTrue.

This was erroneously set to `SetFalse`, which means it's impossible to
enable the terrain from the cli.

src/args.rs

@@ -42,7 +42,7 @@ pub struct Args {
     pub winter: bool,
 
     /// Enable terrain (optional)
-    #[arg(long, default_value_t = false, action = clap::ArgAction::SetFalse)]
+    #[arg(long, default_value_t = false, action = clap::ArgAction::SetTrue)]
     pub terrain: bool,
 
     /// Enable debug mode (optional)