main/postmarketos-base: update motd for the cheatsheet

Adds a link for the cheatsheet replacing the original wiki homepage link. This is for https://gitlab.com/postmarketOS/pmaports/-/issues/972 and depends on https://gitlab.com/postmarketOS/postmarketos.org/-/merge_requests/97 to end up on https://wiki.postmarketos.org/wiki/Terminal_cheat_sheet