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.

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• Added tests.
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• 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.