Fix player number in example game in the ecs_guide (bevyengine#5098)

# Objective

- Small bug in the example game given in examples/ecs/ecs_guide

Currently, if there are 2 players in this example game, the function exclusive_player_system can add a player with the name "Player 2". However, the name should be "Player 3". This PR fixes this. I also add a message to inform that a new player has arrived in the mock game.

Co-authored-by: Dilyan Kostov <dilyanks@amazon.com>

examples/ecs/ecs_guide.rs

@@ -211,9 +211,10 @@ fn exclusive_player_system(world: &mut World) {
     };
     // Randomly add a new player
     if should_add_player {
+        println!("Player {} has joined the game!", total_players + 1);
         world.spawn().insert_bundle((
             Player {
-                name: format!("Player {}", total_players),
+                name: format!("Player {}", total_players + 1),
             },
             Score { value: 0 },
         ));