Fix Rubocop style

[Lyams]

Rubocop test is failed. I fixed style.

https://github.com/butteff/is_dark_ruby_gem/actions/runs/13080776520/job/36503645806:

Inspecting 5 files
..C..

Offenses:

lib/is_dark.rb:110:34: C: [Correctable] Layout/SpaceAroundOperators: Surrounding space missing for operator /.
      percent_calculated = points/(dots.length / 100)
                                 ^

5 files inspected, 1 offense detected, 1 offense autocorrectable