revert maze reward function

[Kallinteris-Andreas]

Description

fixes the change of reward function in #153

Type of change

• Bug fix (non-breaking change which fixes an issue)

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]

Thanks!

[rodrigodelazcano]

@Kallinteris-Andreas can you add these changes to the new maze_v4.py as well?