Wrong documentation for order utility

Created by: hultberg

Hello,

I noticed while reading about the flexbox order utility that the docs state there are several responsive utility classes, like: .order-first, .order-last and so on. However, the actual classes are named .flex-first and .flex-last. The doc in question: https://v4-alpha.getbootstrap.com/utilities/flexbox/#order

Opened a PR correcting the docs: #21811

I'm not sure if this is an issue with the docs, in case .order-* was ment to be the actual classes or that the docs are in fact wrong. 😄