Single line praefect repository migration command

During a Gitaly demo, we noticed copy pasting the curl command in to shell and replacing the private token was more difficult than necessary due to the command being multiline. This commit single lines the multiline curl command.

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