Fix a typo in melatonin code (#4211)

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## About The Pull Request

Melatonin would heal you if you had Burn or Brute damage but harm you if
you had both. It now heals you in both case.

## Why It's Good For The Game

No more random damage.

## Changelog

:cl:
fix: Melatonin being harmfull in some edge case.
/:cl:

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Co-authored-by: ritorizo <ritorizo@localhost>

code/modules/reagents/chemistry/reagents/medicine_reagents.dm

@@ -2304,9 +2304,9 @@
 		if(M.IsSleeping())
 			if(M.getBruteLoss() && M.getFireLoss())
 				if(prob(50))
-					M.adjustBruteLoss(0.5)
+					M.adjustBruteLoss(-0.5)
 				else
-					M.adjustFireLoss(0.5)
+					M.adjustFireLoss(-0.5)
 		else if(M.getBruteLoss())
 			M.adjustBruteLoss(-0.2)
 		else if(M.getFireLoss())