No robots was displayed on the grid, it's now fix

src/main.rs

@@ -253,8 +253,9 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
     let raw_conf = open_file(matches.value_of("file").unwrap_or("two_robots.txt"))?;
 
     let mut robot_pool: Vec<robot::Robot> = Vec::new();
-    let world: world::World = match matches.is_present("random-world") {
-        false => parse_config(raw_conf, &mut robot_pool)?,
+    let mut world: world::World = parse_config(raw_conf, &mut robot_pool)?;
+    world = match matches.is_present("random-world") {
+        false => world,
         true => world::random_world(),
     };