fix: Fix rubocop offenses

[toshimaru]

follow-up to #517.

Error

Offenses:

lib/jekyll-seo-tag/drop.rb:55:11: E: [Correctable] Layout/IndentationWidth: Use 2 (not -12) spaces for indentation. ([https://rubystyle.guide#spaces-indentation](https://rubystyle.guide/#spaces-indentation))
          title + TITLE_SEPARATOR + title_category
          ^^^^^^^^^^^^
lib/jekyll-seo-tag/drop.rb:56:9: C: [Correctable] Layout/ElseAlignment: Align else with if.
        else
        ^^^^
lib/jekyll-seo-tag/drop.rb:58:9: E: [Correctable] Layout/EndAlignment: end at 58, 8 is not aligned with if at 54, 22.
        end
        ^^^

19 files inspected, 3 offenses detected, 3 offenses auto-correctable
Error: Process completed with exit code 1.

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