docs(website): fix typo for onBrokenMarkdownImages

[ethanppl]

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

In the documentation and release notes of 3.9.1, some part referencing the new onBrokenMarkdownImages is written as onBrokenMarkdownLinks.

Test Plan

Tried to search and fixed all the typo I can find. Build locally and tested locally:

Test links

Deploy preview: https://deploy-preview-11444--docusaurus-2.netlify.app/

Related issues/PRs

Related to changes introduced in #11283

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[slorber]

Good catch thanks!