combat_simulator.php: fix wrong variable name

Probably a copy+paste error between the attacker and defender code.

Fixes the phpstan warning:

> Variable $attackerName might not be defined.

src/engine/Default/combat_simulator.php

@@ -42,7 +42,7 @@
 			$duplicates = true;
 			continue;
 		}
-		$usedNames[$attackerName] = true;
+		$usedNames[$defenderName] = true;
 		$defenders[$i] =& DummyPlayer::getCachedDummyPlayer($defenderName);
 		$defenders[$i]->setAllianceID(2);
 		$realDefenders[$i] =& $defenders[$i];