doc: fix mermaid diagram

[holmanb]

Proposed Commit Message

docs: fix mermaid diagram

It seems a recent mermaid release was not backwards compatible
with the diagram syntax. Fix it.

Fixes GH-6668

Additional Context

Fixes #6668

Test Steps

Merge type

• Squash merge using "Proposed Commit Message"
• Rebase and merge unique commits. Requires commit messages per-commit each referencing the pull request number (#<PR_NUM>)