Fix AD_DCAY mhitm armor erosion type

doc/fixes37.0

@@ -318,6 +318,8 @@ reduce the number of "seeXYZ" commands by renaming some: #seenv -> #wizseenv,
 	#seegold -> #showgold, #seespells -> #showspells, #seetrap -> #showtrap
 when saving while punished or game ends while punished, handling for ball and
 	chain might access freed memory with unpredictable consequences
+brown pudding monster hitting another monster with decay attack corroded armor
+	instead of rotting it
 
 
 Fixes to 3.7.0-x Problems that Were Exposed Via git Repository

src/uhitm.c

@@ -1826,7 +1826,7 @@ struct mhitm_data *mhm;
             mhm->hitflags = (MM_DEF_DIED | (grow_up(magr, mdef) ? 0 : MM_AGR_DIED));
             return;
         }
-        erode_armor(mdef, ERODE_CORRODE);
+        erode_armor(mdef, ERODE_ROT);
         mdef->mstrategy &= ~STRAT_WAITFORU;
         mhm->damage = 0;
     }