Fix signatures of rising_factorial() and falling_factorial()

[mcol]

Submission Checklist

• Builds locally
• Declare copyright holder and open-source license: see below

Summary

Fixes #93.

Copyright and Licensing

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Marco Colombo

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• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[bob-carpenter]

Thanks!