Fix example in Rails/PluralizationGrammar

[masato-bkn]

I fixed the example in Rails/PluralizationGrammar.

---

Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• 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.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
  * [ ] Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.
  * [ ] If this is a new cop, consider making a corresponding update to the Rails Style Guide.

[koic]

Good catch!