Correct capture_body config var values referred to in messaging spec

[trentm]

A small thing I noticed when reading through the messaging spec, in prep for span link spec work.

This is a small enhancement

• [x] Create PR as draft
• [x] Approval by at least one other agent
• [x] Mark as Ready for Review (automatically requests reviews from all agents and PM via CODEOWNERS)
  • Remove PM from reviewers if impact on product is negligible
  • Remove agents from reviewers if the change is not relevant for them
• [ ] ~Merge after 2 business days passed without objections~ This is just a small correction.

[apmmachine]

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#### Build stats * Start Time: 2022-03-28T04:55:00.839+0000 * Duration: 3 min 6 sec