Substantially increased the chance of the experimental fighter appearing.

It previosly spawned half of the time. Now it spawns 4/5 of the time.
This makes waiting for it to show up so you can fight it a lot less
painful, even if it doesn't make a whole lot of sense.

src/game.cpp

@@ -370,7 +370,7 @@ int mainGameLoop()
 
 		if ((currentGame.system == 2) && (currentGame.experimentalShield > 0))
 		{
-			if ((rand() % 2) == 0)
+			if ((rand() % 5) < 4)
 			{
 				aliens[10] = alien_defs[CD_CLOAKFIGHTER];
 				aliens[10].owner = &aliens[10];