Hotfix for Vecnormalize

[araffin]

Description

Spotted by @vwxyzjn in openai/gym#2387

Motivation and Context

• I have raised an issue to propose this change (required for new features and bug fixes)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (update in the documentation)

Checklist:

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• My change requires a change to the documentation.
• I have updated the tests accordingly (required for a bug fix or a new feature).
• I have updated the documentation accordingly.
• I have reformatted the code using make format (required)
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[Miffyli]

One small comment but other than that LGTM! Feel free to approve and merge after addressing the comment :)

[Miffyli]

LGTM!