[Filebeat] httpjson update docs

[leehinman]

What does this PR do?

Updates documentation for accessing last_response.url.params in httpjson input.

Why is it important?

Previous documentation had params as a map which wasn't correct and lead to confusion about accessing the values in the params.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Related issues

• Relates [Filebeat] httpjson last_response url parameters not correct. #30477

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Pinging @elastic/security-external-integrations (Team:Security-External Integrations)

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Build stats

• Start Time: 2022-03-10T15:58:14.027+0000

• Duration: 20 min 39 sec

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[andrewkroh]

LGTM

[mergify]

This pull request is now in conflicts. Could you fix it? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b httpjson_params_doc_update upstream/httpjson_params_doc_update
git merge upstream/main
git push upstream httpjson_params_doc_update