Fix DoubleStartEndWith example

[catlee]

The example code for the DoubleStartEndWith is missing the # bad header for the first section here: https://docs.rubocop.org/rubocop-performance/cops_performance.html#includeactivesupportaliases-false-default

This PR adds the appropriate comments for this example.

Before submitting the PR make sure the following are checked:

• [x] The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• [x] Wrote good commit messages.
• [x] Commit message starts with [Fix #issue-number] (if the related issue exists).
• [x] Feature branch is up-to-date with master (if not - rebase it).
• [x] 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.

[koic]

This seems to be an expected behavior of IncludeActiveSupportAliases: false. Accepting the changes in this PR would make IncludeActiveSupportAliases: false and IncludeActiveSupportAliases: true behave the same.