fix arrow type

src/main.rs

@@ -17,10 +17,10 @@ impl World {
     /// Set robot on the map.
     fn set_robot(&mut self, r: Robot) {
         self.map[(r.p.x * r.p.y) as usize] = match r.o {
-            Orientation::N => '⬆',
+            Orientation::N => '↑',
-            Orientation::S => '⬇',
+            Orientation::S => '↓',
-            Orientation::E => '➡',
+            Orientation::E => '→',
-            Orientation::W => '⬅',
+            Orientation::W => '←',
         }
     }
 }