chore: fix RST syntax error

[brobro10000]

Description:
This pull request is a minor release update for the openedx_ledger package. The version has been incremented to 1.6.15, and the CHANGELOG.rst formatting has been updated for recent releases.

Release management:

• Updated the version number in openedx_ledger/__init__.py to 1.6.15.

Changelog formatting:

• Increased the number of asterisks in the section headers for recent releases in CHANGELOG.rst to standardize formatting.

Testing instructions:
Add some.

Merge checklist:

• All reviewers approved
• CI build is green
• Version bumped
• Changelog record added
• Documentation updated (not only docstrings)
• Commits are squashed

Post merge:

• Create a tag
• Check new version is pushed to PyPI after tag-triggered build is
  finished.
• Delete working branch (if not needed anymore)