Fixes entry_point and kwargs in isaaclab_tasks README

[fan-ziqi]

Description

The entry_point RLTaskEnv doesn't exist

It should be ManagerBasedRLEnv

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