Fix doc get_custom_rewards()

[Xakka]

Pull request summary

Fixed formatting Added Return

Checklist

• [ ] If code changes were made then they have been tested.
  • [ ] I have updated the documentation to reflect the changes.
  • [ ] I have updated the changelog with a quick recap of my changes.
• [ ] This PR fixes an issue.
• [ ] This PR adds something new (e.g. new method or parameters).
• [ ] This PR is a breaking change (e.g. methods or parameters removed/renamed)
• [x] This PR is not a code change (e.g. documentation, README, ...)