update relx.config vm.args copy

Recent relx changed the output structure to be releases/<rel_version> instead of releases/<rel_name>-<rel_version> in order to work with release_handler. Here I've made the change to relx.config overlay for that.

You could also use 

```
{vm_args, "rel/vm.args"}.
```

Instead, like you do for `sys.config`.