Fix vision notebook links and RSL-RL log path in learning README

[rishabhsai]

The two Colab links at the bottom of the tutorial list in learning/README.md are dead. They point at learning/notebooks/training_vision_1.ipynb and training_vision_2.ipynb, which were deleted in 903fcd7 and replaced with the single learning/notebooks/vision.ipynb; that commit updated the notebook table in the top-level README but not this file, so both links have returned 404 since. The RSL-RL section also said logs and checkpoints go to a logs directory, which is true of train_jax_ppo.py but not of train_rsl_rl.py, where line 144 builds logdir from /tmp/rslrl-training-logs/ plus the experiment name and line 213 resolves --load_run_name against the same prefix. I replaced the two dead items with one "Vision Environments" item using the top-level README's wording, renumbered, and corrected the log path sentence; curl -sIL returns 200 for vision.ipynb and 404 for both training_vision_*.ipynb, and grep -n training_vision learning/README.md is now empty.