docs: fix mermaid theme options typo

[ntucker]

According to the types here https://github.com/facebook/docusaurus/blob/9c92a79d23533a61f3ad1bf3ea4d0889f0e2710f/packages/docusaurus-theme-mermaid/src/theme-mermaid.d.ts as well as runtime validation by myself - it should options not mermaidOptions.

Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.
• If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
• If this is a new API or substantial change: the PR has an accompanying issue (closes #0000) and the maintainers have approved on my working plan.

Motivation

Docs are misleading

Test Plan

Set some options using this method and they applied

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[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

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[slorber]

Thanks, missed that 👍 backported to 2.2 docs too