fix truncarted typo in readme API example

[rdnfn]

Description

This tiny PR removes one character from the readme in order to fix a typo in the API example. The typo is that truncarted was written instead of truncated. Without this fix the example does not work (in Gym v0.26) as it throws a NameError: name 'truncated' is not defined. Also, thanks for all the awesome work!

Type of change

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

Screenshots

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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