Workaround for broke CI with YARD 0.9.20

[koic]

This PR solves the following "The manual directory is out of sync" error.

% ruby -v
ruby 2.7.0dev (2019-12-03T05:51:14Z master e42d9d8df8) [x86_64-darwin17]
% CI=true bundle exec rake

(snip)

The manual directory is out of sync. Run `rake
generate_cops_documentation` and commit the results.

https://circleci.com/gh/rubocop-hq/rubocop-rails/2180

This issue will be resolved with the release of YARD 0.9.21 or higher, which includes the following changes:
lsegal/yard#1290

---

Before submitting the PR make sure the following are checked:

• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Run bundle exec rake default. It executes all tests and RuboCop for itself, and generates the documentation.