Fix message center api spectral errors

[Isabella-Veloso]

Fixing Message Center API spectral errors.

Types of changes

• [ ] New content (endpoints, descriptions or fields from scratch)
• [x] Improvement (make an endpoint's title or description even better)
• [ ] Spelling and grammar accuracy (self-explanatory)

Changelog

Do not forget to update your changes to our Developer Portal's changelog. Did you create a release note?

• [ ] Yes, I already created a release note about this change.
• [ ] No, but I am going to.

[github-actions[bot]]

Thanks for your contribution. The .json file will be checked now with Spectral.