Doc: Troubleshoot docs-ci failure related to kafka output docs

[karenzone]

Docs-ci failure shows up in this PR: https://github.com/elastic/logstash-docs/pull/1625

Caused by: Mismatch between TOC entry and anchor defined in heading.

Proposed resolution

Standardize on plural to match heading name and defined anchor and usage in code. This means updating the TOC entry and code snippet to message_headers (plural).

[karenzone]

NOTES for future travelers:

• I start by fixing the problem in logstash-docs (https://github.com/elastic/logstash-docs/pull/1625)in order to:
  • 1.) be sure that my theory pans out and that there are no futher build errors
    (Remember, docs-ci stops processing when it hits the first error.)
  • 2.) get the Versioned Plugin Reference (VPR) to pass docs-ci so that we can get plugin docs building again.
• Next, formatting problems need to be fixed in the source (which I will do shortly).
• Depending on the plugin gemlock files, we'll need to fix this in the elasticsearch-output for each branch (main,8.13, possibly others.