Updating Dataset Reproducibility Guide with mujoco environment versions

[micahcarroll]

Currently the reproducibility guide in the Mujoco section doesn't contain information about which version of the environments were used (Hopper-v2 vs Hopper-v3), nor this is present anywhere else in the codebase.

I contacted the authors and they replied the version should be -v2 across all Mujoco tasks – this is useful for keeping the dataset environment and the testing environment consistent at evaluation time. This info should probably be added to the Wiki.