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

[fan-ziqi]

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

• I have run the pre-commit checks with ./isaaclab.sh --format
• I have made corresponding changes to the documentation
• 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

[ooctipus]

@fan-ziqi Thanks for the PR : ))