fix: build failure dure to incorrect type

[saurabhraghuvanshii]

Description

Updated Gatsby’s MDX frontmatter schema to fix course page build failures by changing toc from Boolean to [String] in gatsby-node.js.

This aligns GraphQL typing with existing course MDX content (toc as chapter slug arrays), resolving the Boolean cannot represent a non boolean value error during course-overview query execution.

This PR fixes #

Notes for Reviewers

Signed commits

• Yes, I signed my commits.

[leecalcote]

Incorrect type of what?

[leecalcote]

PR descriptions... don't forget those (as you have been).

[arjunmehta-git]

Thanks for the quick turnaround on this, @saurabhraghuvanshii. A couple of things that will help reviewers move fast on this:

1. Description is empty — The PR body still has the placeholder text. A one-line explanation of what type was wrong and in which context (e.g., "gatsby-node.js was passing a string where an integer was expected for X") goes a long way.
2. Issue reference — The "This PR fixes #" line is blank. If this relates to a filed issue, link it; if it's a drive-by fix, just remove the line so reviewers aren't hunting for a phantom issue.
3. Commit sign-off — Please confirm your commit is signed off (checkbox is unchecked in the template).

The change to gatsby-node.js is minimal and easy to review once we have that context. Looking forward to getting this in.

[saurabhraghuvanshii]

@leecalcote @arjunmehta-git Done !! Please re-review