Fixing broken url for Breaking changes in 7.0 in docs.

[Pro777]

elasticsearch/reference/current url now references Elasticsearch 8.0, which doesn't include the guide on breaking changes in the 7.0 version.

I've adjusted the URL to point to the correct location in elasticsearch/reference/7.17

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• 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.
• Added an entry to the changelog if the new code introduces user-observable changes. See changelog entry format for details.

[rabotyaga]

👋 @Pro777
Thanks!