Update "What is a pattern?"

I noticed some of the type definitions described in ["What is a pattern?"][1] don't match up with the source code anymore. I'm not familiar with Haskell but it seems types changed significantly around https://github.com/tidalcycles/Tidal/commit/44adf93f84a25d72a6b4540770f7abbf8d8099b5.

[1]: https://tidalcycles.org/docs/innards/what_is_a_pattern