Remove RHTML reference in Action Controller docs

[lachlanjc]

This PR fixes the text in the Action Controller docs, where it mentions that the static error pages don't use RHTML.

Original:

You can customize these files to add some extra information and layout, but remember that they are static; i.e. you can't use RHTML or layouts in them, just plain HTML.

Revised:

You can customize these files to add some extra information and style, but remember that they are static HTML; i.e. you can't use ERB, SCSS, CoffeeScript, or layouts for them.

[rails-bot]

Thanks for the pull request, and welcome! The Rails team is excited to review your changes, and you should hear from @eileencodes (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

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[eileencodes]

Thanks for the PR @lachlanjc! Can you squash your commits into one commit? Here's some info on how to do that: https://git-scm.com/book/en/v2/Git-Tools-Rewriting-History 😄

[lachlanjc]

@eileencodes Ok! I just removed them both and re-committed. How do my changes look?

[eileencodes]

Thanks for the quick update!

[lachlanjc]

And for the quick review! 💥🙌😉