Fix json_scanf example

Resolves https://github.com/cesanta/frozen/issues/13

PUBLISHED_FROM=e6a9f5c81c3882bc8563d22fbf87caa3388ca1c8

docs/examples/json_scanf.md

@@ -8,8 +8,9 @@ title: json_scanf() example
 
   int a, b;
   char *c;
+  void *my_data = NULL;
   json_scanf(str, strlen(str), "{ a:%d, b:%M, c:%Q, d:%B }",
-             &a, &b, &c, scan_array, my_data);
+             &a, scan_array, my_data, &c, &b);
 
 
   // This function is called by json_scanf() call above.