Update README.md to work on PyPi

[LukeWood]

Love the package, am using it in a toy RL project. Love mario and love RL so this is great!

The link to actions in the PyPi readme is broken right now: https://pypi.org/project/gym-super-mario-bros/

Check it out - this fixes it.

Cheers mate!

Description

Please include a summary of the change and which issue is fixed. Please also include relevant motivation and context. List any dependencies that are required for this change.

• Fixes #

Type of change

Please select all relevant options:

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] MGMT (non-breaking change to deployment, CI, etc.

Checklist

• [ ] My code follows the style guidelines of this project
• [ ] I have performed a self-review of my own code
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [ ] I have added tests that prove my fix is effective or that my feature works

[Kautenja]

Thanks!