Fixes symbol in training_jetbot_reward_exploration.rst (#2722)

# Description

Just fixed a doc formatting issue (an extra space prevents proper syntax
highlighting on this documentation page).

## Type of change

- Bug fix (non-breaking change which fixes an issue)

## Checklist

- [ ] I have run the [`pre-commit` checks](https://pre-commit.com/) with
`./isaaclab.sh --format`
- [x] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [ ] I have updated the changelog and the corresponding version in the
extension's `config/extension.toml` file
- [ ] I have added my name to the `CONTRIBUTORS.md` or my name already
exists there
Signed-off-by: Doug Fulop <dougfulop@gmail.com>