fix: malformed code snippet in integrations-guide/mdx.mdx

[ArmandPhilippot]

Description (required)

• The code snippet in the Using MDX with content collections section had an unclosed function, so I fixed it
• There was a typo in the section heading wth > with

I don't know if other languages ​​than Korean are already up to date on this file, so I didn't included the changes on the Korean version.

Related issues & labels (optional)

• Suggested label: typo, code snippet update

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🔄️	The source for this localization has been updated since the creation of this pull request, make sure all changes in the source have been applied.

[thomasbnt]

LGTM!

[yanthomasdev]

LGTM! Thanks!