fix: Update FlatCompat docs + typings to reflect Array

[wesbos]

I know this repo is frozen, but I'm just trying out 1.4 and all the compat methods give me an array, passing them into the export default [] as shown in the docs gives "Unexpected Array" Error.

Perhaps there is just a missing .flat() on the end of the config instead of having to spread into the config array?

[eslint-github-bot]

Hi @wesbos!, thanks for the Pull Request

The pull request title isn't properly formatted. We ask that you update the message to match this format, as we use it to generate changelogs and automate releases.

• The commit message tag wasn't recognized. Did you mean "docs", "fix", or "feat"?
• There should be a space following the initial tag and colon, for example 'feat: Message'.
• The first letter of the tag should be in lowercase

To Fix: You can fix this problem by clicking 'Edit' next to the pull request title at the top of this page.

Read more about contributing to ESLint here

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: wesbos / name: Wes Bos (7a41c41, aa12328)

[nzakas]

Yes! Thank you. This was on my list to get back to fixing.