Correct point_obs slicing for achieved_goal in point_maze environments

[dohmjan]

Description

Hello!

Right now achieved_goal = point_obs[2:] refers to velocities while it should be x, y positions instead (desired_goal is a position).
point_obs is eventually coming from np.concatenate([self.data.qpos, self.data.qvel]).ravel() in the PointEnv.

This PR corrects the slicing s.t. achieved_goal = point_obs[:2] refers to x, y positions.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• I have run the pre-commit checks with pre-commit run --all-files (see CONTRIBUTING.md instructions to set it up)
• I have commented my code, particularly in hard-to-understand areas
• 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
• New and existing unit tests pass locally with my changes

[rodrigodelazcano]

Thaks @dohmjan! you are right about this