fix chefspec depreciation warning about `should`

Trivial fix for the following warning:

```

Deprecation Warnings:

Using `should` from rspec-expectations' old `:should` syntax without explicitly
enabling the syntax is deprecated. Use the new `:expect` syntax or explicitly enable `:should` instead.
Called from […]/chef-os-hardening/spec/recipes/default_spec.rb:35:in `block (2 levels) in <top (required)>'.


If you need more of the backtrace for any of these deprecations to
identify where to make the necessary changes, you can configure
`config.raise_errors_for_deprecations!`, and it will turn the
deprecation warnings into errors, giving you the full backtrace.
```