Change level difficulty formula for the ring of aggravate monster

+15 wasn't very impactful in the late game and late mid-game, but
was much too lethal in the early game (wearing the ring for a while
near the start of the game would make the game unwinnable as very
out-of-depth monsters spawned, and they would still be there even
after removing the ring and usually capable of one-shotting an
early-game character). This commit changes it to a doubling of
level difficulty rather than a flat increase: that makes it more
relevant in the late-game where a +25 or +50 might potentially have
an impact, and more survivable in the early game (although it still
spawns monsters that are difficult for the point in the game, there
is now a chance that you might survive long enough to be able to
take the ring off and clear off all the out-of-depth monsters that
spawned).

src/dungeon.c

@@ -2073,7 +2073,7 @@ level_difficulty(void)
     }
     /* ring of aggravate monster */
     if (EAggravate_monster)
-        res += 15;
+        res = res > 25 ? 50 : res * 2;
     return res;
 }