Fix an auto-correction bug in Lint/BooleanSymbol

[knu]

It replaces a boolean symbol in a hash value as if it were a key, generating a broken piece of code.

# before
{ null: :false }

# after: wrong (current behavior)
{ null :false => }

# after: correct (fixed behavior)
{ null: false }

# before
{ false: :false }

# after: wrong (current behavior)
{ false => :false => }

# after: correct (fixed behavior)
{ false => false }

Replace this text with a summary of the changes in your PR.
The more detailed you are, the better.

---

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.

[koic]

This looks good to me. Can you squash your commits into one?

[knu]

@koic I just felt like it would create a time paradox to include a change log update with a PR number in a single commit, but I will if you say so. 😆

[koic]

Oops, I overlooked one. Can you enclose the cop name in backticks?

Suggested change

	* [#7871](https://github.com/rubocop-hq/rubocop/pull/7871): Fix an auto-correction bug in Lint/BooleanSymbol. ([@knu][])
	* [#7871](https://github.com/rubocop-hq/rubocop/pull/7871): Fix an auto-correction bug in `Lint/BooleanSymbol`. ([@knu][])

[knu]

Done!

[koic]

Thank you!