Fix escape character warning in migration

[gerrod3]

/src/pulpcore/pulpcore/app/migrations/0155_create_rel_path_domains.py:7: SyntaxWarning: invalid escape sequence '\s'
  CREATE DOMAIN "relative_path" AS text CHECK ('/' || VALUE || '/' !~ '[\n\r\s\t\?#]|(/\.{0,2}/)');

Set the SQL string to raw to avoid warning when file is loaded.

📜 Checklist

• Commits are cleanly separated with meaningful messages (simple features and bug fixes should be squashed to one commit)
• A changelog entry or entries has been added for any significant changes
• Follows the Pulp policy on AI Usage
• (For new features) - User documentation and test coverage has been added

See: Pull Request Walkthrough