doc: `write:org` needed for groupsCacheTTL service token

Created by: bobheadxi

An error was encountered where:

suggestion="GitHub access token user may only have read access to the repository, but needs write for permissions"

on an API request for a repo's /collaborators?page=1&per_page=100&affiliation=direct

It seems this indeed might be the case - to see the collaborators of a repo, you must be able to edit collaborators as well? I can't find anything on a cursory skim through our other docs, but this updates it for groups caching at least