fix: incorrect exception handling in factorial function

[henriq4]

Description of Change

This pull request corrects the recursive factorial function by:

• Fixing the incorrect exception handling (previously throwing an exception for values less than 20 instead of greater than 20).
• Ensuring that the function throws an exception when n > 20 to prevent overflow in uint64_t.

Checklist

• Added description of change
• Added file name matches File name guidelines
• Added tests and example, test must pass
• Added documentation so that the program is self-explanatory and educational - Doxygen guidelines
• Relevant documentation/comments is changed or added
• PR title follows semantic commit guidelines
• Search previous suggestions before making a new one, as yours may be a duplicate.
• I acknowledge that all my contributions will be made under the project's license.

Notes: This is my first good issue, and I’m grateful to contribute to the project. Fixes #2867

[realstealthninja]

nice spotting! thank you for this.