Env creation tutorial 3: Change action mask type to int8

[BertrandDecoster]

Description

The expected type of an action mask in gymnasium is int8 (cf https://github.com/Farama-Foundation/Gymnasium/blob/40822a9e8fabb5b064597aaf1982f997162bd369/gymnasium/spaces/discrete.py#L58)

assert (
                mask.dtype == np.int8
            ), f"The expected dtype of the mask is np.int8, actual dtype: {mask.dtype}"

This PR just changes the type of the mask to the right type

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• 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 run pytest -v and no errors are present.
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I solved any possible warnings that pytest -v has generated that are related to my code to the best of my knowledge.
• 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

It's just a doc change