Fix the file lookup in syntax toggles

[remcohaszing]

Initial checklist

• I read the support docs
• I read the contributing guide
• I agree to follow the code of conduct
• I searched issues and couldn’t find anything (or linked relevant results below)
• If applicable, I’ve added docs and tests

Description of changes

Without this fix, the syntax toggles didn’t do anything at all.

[changeset-bot]

🦋 Changeset detected

Latest commit: 2cc4422

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 3 packages

Name	Type
@mdx-js/language-service	Patch
@mdx-js/language-server	Patch
@mdx-js/typescript-plugin	Patch

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[github-actions]

Hi! This is accepted and can go somewhere!

Team: please review this PR and use the area/* (to describe the scope of the change), platform/* (if this is related to a specific one), and semver/* and type/* labels to annotate this.

[github-actions]

Hi! This was closed. Team: If this was merged, please describe when this is likely to be released. Otherwise, please add one of the no/* labels.