Lint/ToJSON suggest wrong code

[mrasu]

Expected behavior

With following suggestion at Lint/ToJSON, not raise error.
https://github.com/rubocop-hq/rubocop/blob/master/lib/rubocop/cop/lint/to_json.rb#L17

@example
  # bad
  def to_json
  end

  # good
  def to_json(_opts)
  end

Actual behavior

Because I get Lint/ToJSON, I changed my code like below

# Before
class Ex
  def to_json
    {a: 1}.to_json
  end
end

# After
class Ex
  def to_json(_opts)
    {a: 1}.to_json
  end
end

Then, Ex.new.to_json start raising error saying ArgumentError: wrong number of arguments (given 0, expected 1)

As talked #6379 (comment), document should suggest the argument to be optional

RuboCop version

$ bundle exec rubocop -V
0.66.0 (using Parser 2.6.0.0, running on ruby 2.4.1 x86_64-linux)

[koic]

I think it was a mistake of multiple arguments. I opened a PR #6859.