Fix requirements

[hpesonen]

Summary:

Fix a typo in requirements

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• You have read contribution guidelines
• You have updated CHANGELOG.rst
• You have listed the copyright holder for the work you are submitting (see next section)

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• You have included or updated all the relevant documentation, including docstrings
• You have added appropriate functional and unit tests to ensure the new features behave as expected
• You have run make lint, make docs and make test

and the proposed changes pass all unit tests (check step 6 of CONTRIBUTING.rst for details)

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