Fix typo for header in repo migrate API

What does this MR do?

Corrects a typo that prevents correct usage of the repo migration API.

The correct usage is documented here: https://docs.gitlab.com/ee/api/README.html#personalproject-access-tokens

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content