Fix unit of marketPlace plans

[blacelle]

Description

This fixes an issue reported in #1613, where marketPlace plans type are not properly described (e.g. per-unit instead of PER_UNIT). This is confirmed by Github documentation https://docs.github.com/en/rest/apps/marketplace?apiVersion=2022-11-28#list-plans

Fixes #1613

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[blacelle]

The new syntax is confirmed by https://github.com/hub4j/github-api/pull/1630/files#diff-972673bd4c7fcae9ab51380063109324d7774c101989182253b980165243275aR24, which is a recent recording.

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (600933b) 79.64% compared to head (cb089b9) 79.64%.

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #1629   +/-   ##
=========================================
  Coverage     79.64%   79.64%           
  Complexity     2187     2187           
=========================================
  Files           208      208           
  Lines          6657     6657           
  Branches        364      364           
=========================================
  Hits           5302     5302           
  Misses         1141     1141           
  Partials        214      214           

Impacted Files	Coverage Δ
...va/org/kohsuke/github/GHMarketplacePriceModel.java	87.50% <100.00%> (ø)

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[bitwiseman]

I've manually verified that this change matches the documentation.