Adjust example to show how other common directives can be used

[dwightwatson]

I've been playing with the cache.headers middleware and exploring s-maxage and stale-while-revalidate directives.

While the paragraph above does spell out that Directives should be provided using the "snake case" equivalent of the corresponding cache-control directive, I still missed the subtle difference of hyphen placement between max-age and s-maxage and was running into exceptions like Response does not support the following options: "smax_age"..

I thought it might improve discoverability to include these common directives in the example. If you're exploring how to use cache headers you should be familiar with the options. I know this would have helped me.