Fix a typo in the UG_USE_SYSTEM_HEAP macro

I had simply misspelled it in the DisposeMem method.
It's a surprise the program never crashed with such a severe
bug in it.

low/heaps.c

@@ -461,7 +461,7 @@ void *NS_PREFIX GetMemUsingKey (HEAP *theHeap, MEM n, HeapAllocMode mode, INT ke
 
 void NS_PREFIX DisposeMem (HEAP *theHeap, void *buffer)
 {
-#if US_USE_SYSTEM_HEAP
+#if UG_USE_SYSTEM_HEAP
   free(buffer);
 #else
   BLOCK *newBlock,*theBlock,*nextBlock;