fix output of size class offset test

test/Makefile

@@ -0,0 +1,7 @@
+EXECUTABLES := \
+    offset
+
+all: $(EXECUTABLES)
+
+clean:
+	rm -f $(EXECUTABLES)

test/offset.c

@@ -30,7 +30,7 @@ int main(void) {
             return 1;
         }
         if (i != 0) {
-            printf("%zu to %zu: %zd\n", size_classes[i - 1], size, p[i - 1] - p[i]);
+            printf("%zu to %zu: %zd\n", size_classes[i - 1], size, p[i] - p[i - 1]);
         }
         printf("%zu to %zu: %zd\n", size, size, q - p[i]);
     }