fix case of asset for linux

CatapultNetWars/Players/Human.cs

@@ -79,7 +79,7 @@ namespace CatapultGame
 
 		public override void Initialize ()
 		{
-			arrow = curGame.Content.Load<Texture2D> ("Textures/HUD/arrow");
+			arrow = curGame.Content.Load<Texture2D> ("Textures/HUD/Arrow");
 
 			Catapult.Initialize ();