[Schema Inaccuracy] `PUT /repos/{owner}/{repo}/collaborators/{username}`: `permission` parameter & custom role names

[gr2m]

See https://github.com/github/rest-api-description/pull/561/files#r714162574

@imwiss the description addition says that permission can be set to a custom role name

custom repository role name - Can assign a custom repository role if the owning organization has defined any.

But the property has an enum that does not permit custom values. It only permits

* `pull`
* `push`
* `admin`
* `maintain`
* `triage`