Add syntax highlighting to README

[ydah]

Follow up: https://github.com/rubocop/rubocop-rspec/pull/1641

---

Before submitting the PR make sure the following are checked:

• [x] Feature branch is up-to-date with main (if not - rebase it).
• [x] Squashed related commits together.
• [-] Added tests.
• [x] Updated documentation.
• [-] Added an entry to the CHANGELOG.md if the new code introduces user-observable changes.
• [x] The build (bundle exec rake) passes (be sure to run this locally, since it may produce updated documentation that you will need to commit).

[pirj]

Thank you!