Changes the expected input of checkpoint in rsl-rl to an absolute path in documentation to avoid ambiguity (#3151)

# Description

If the user directly enters model_xxx.pt, an error will be reported. The
user should be guided to enter a path

## Type of change

- This change requires a documentation update

## Checklist

- [x] 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
- [x] I have added my name to the `CONTRIBUTORS.md` or my name already
exists there
Co-authored-by: ooctipus <zhengyuz@nvidia.com>