Standardize curl examples in doc/api/repository_submodules.md

fixes the part of issue #509818

This MR standardizes the curl command examples in the Repository submodules API documentation to follow the updated GitLab documentation guidelines.

Changes:

• Broken the single-line curl command into multiple lines for better readability.
• Split benched parameters (--data) into individual lines.
• Explicitly declared the --url parameter instead of implying it.
• Ensured consistent 2-space indentation for continuation lines.

These changes help reduce horizontal scrolling and improve the maintainability of the documentation.