Change flag color to black banner.

Updated the default flag item from a cyan banner to a black banner.

src/main/java/com/alttd/ctf/flag/Flag.java

@@ -34,7 +34,7 @@ import java.util.stream.Collectors;
 public class Flag implements Runnable {
 
     private final HashMap<Integer, Integer> teamFlagPointCount = new HashMap<>();
-    private final ItemStack flagItem = new ItemStack(Material.CYAN_BANNER);
+    private final ItemStack flagItem = new ItemStack(Material.BLACK_BANNER);
     private final BossBar bossBar = createBossBar();
     private final HashMap<Integer, Integer> wins = new HashMap<>();
     private int lastWinningTeamId = -1;