Fix AdamW weight_decay default value in docstring

[goingreen]

Proposed changes

Fix docstring for AdamW weight_decay default value

Checklist

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• I have read the CONTRIBUTING document
• I have run pre-commit run --all-files to format my code / installed pre-commit prior to committing changes
• I have added tests that prove my fix is effective or that my feature works
• I have updated the necessary documentation (if needed)

[awni]

Good catch, thanks!