[MAINT]: organization custom property resource needs to be rename to singular

[nickfloyd]

Describe the need

The implemented resource for organization custom property uses the plural naming scheme but does not handle collections of the given resource.

The general pattern has been to use plural on collections and singular on single resource operations.

For instance, repository collaborators (single, collection) implements this approach.

We need two things here:

1. We need to rename the resource - BREAKING CHANGE
2. We need to document in our contributor docs this pattern for future work.

SDK Version

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API Version

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