mate.mate-user-guide: 1.20.0 -> 1.20.1 (#42480)

Semi-automatic update generated by https://github.com/ryantm/nixpkgs-update tools.

This update was made based on information from https://repology.org/metapackage/mate-user-guide/versions.

These checks were done:

- built on NixOS

- 0 of 0 passed binary check by having a zero exit code.
- 0 of 0 passed binary check by having the new version present in output.
- directory tree listing: https://gist.github.com/69230e142f75a0bf18525c97d28a5fae
- du listing: https://gist.github.com/82fdc8e01e8e7a7c1fc4250a582e480a