Fix commas in code blocks of markdown

[exclipy]

Description

If a comma appeared in a code block in markdown with the Remark/PrismJS plugin, it was being rendered as the string undefined. This PR fixes that so commas are now rendered as commas.

Documentation

I'll need help with updating the documentation. This bug affects Gatsby's website and it will have to be rebuilt with this fix.

Related Issues

Fixes #20270

[vladar]

Oh, this is a funny bug. Thanks for noticing and fixing it! 💜

[gatsbot]

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Thanks again!

[kinnikumegane]

Hello all,
I have the same issue here using gatsby-v2-starter-lumen template.
When I write comma "," in code blocks of markdown file, it's rendered as "undefined".
Would you know how to fix it ?

How I write in markdown file

resolve: gatsby-plugin-gtag,
options: {
trackingId: process.env.GOOGLE_ANALYTICS_ID,
head: true,
anonymize: true,
}

How it's rendered in html

resolve: gatsby-plugin-gtagundefined
options: {
trackingId: process.env.GOOGLE_ANALYTICS_IDundefined
head: trueundefined
anonymize: trueundefined
}

Thanks a lot for your support !

[MarkosKon]

This PR seems to fix the problem, but there is no version bump. As a result, the fix is not published on npm.

[thaind97git]

@vladar Seems to this PR not yet published, Right?

[vladar]

Published in [email protected]

[kinnikumegane]

Thanks all for paying attention.
I got solution here, so it's fixed now on my end.

Solution;
I specify language "javascript" after three times "`", so that code block understands that it's written in javascript, and comma is rendered as comma.

```javascript

[Bizzaro]

awesome - just what i was looking for.
upgrading to [email protected] fixed it.