Fix compiler warning in Array.take() method (#611)

Fixes one of the compiler warnings mentioned in #610.

src/Array.mo

@@ -859,7 +859,7 @@ module {
     let len = Prim.abs(length);
     let size = array.size();
     let resSize = if (len < size) { len } else { size };
-    let start = if (length > 0) 0 else size - resSize;
+    let start : Nat = if (length > 0) 0 else size - resSize;
     subArray(array, start, resSize)
   }
 }