Unmark AutoCorrect: false from Rails/UniqBeforePluck

[koic]

This AutoCorrect: false looks like it was set when there was no way to safe autocorrect by SafeAutocorrect: false.
Test code for the autocorrection exists. So it can be enabled by default. However, it is still unsafe because SafeAutocorrect: false.

---

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• 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.
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• 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.
• If this is a new cop, consider making a corresponding update to the Rails Style Guide.

[pirj]

Side observation.

Examples are outdated, too.

# this will ~return a Relation~ that pluck is called on
Model.where(cond: true).pluck(:id).uniq

check:

> x=User.where('1 = 1').pluck(:id);
SELECT "users"."id" FROM "users" WHERE (1 = 1)
> x.class
=> Array

uniq will be called on an array.

# an association on an instance will ~return a CollectionProxy~
instance.assoc.pluck(:id).uniq

It will return an Array.

# good
Model.distinct.pluck(:id)

I understand the reasoning, but the usage of id breaks it.

Model.distinct.pluck(:favourite_food)

would make more sense.
I believe the usage of id in those examples is more confusing than helpful.

[koic]

Thanks for the feedback. That point can open a PR separately. Can you open a PR if you like?