While looking on the bug tracker I've found that someone opened an issue for a typo (https://bugs.python.org/issue45370)

There is a typo in https://www.python.org/dev/peps/pep-0636/#composing-patterns

The second sentence in the first paragraph : "[...] we have being doing that implicitly in the examples above."

When it should be "[...] we have been doing that implicitly in the examples above."

Opening an issue here and creating a PR for it.