fix(maths): correct recursive call in sum_of_digits_recursion

[fauzan171]

Summary

Fix a bug where sum_of_digits_recursion() was calling sum_of_digits() (the iterative version) instead of itself in the recursive case.

Bug

In maths/sum_of_digits.py line 34:

# Before (bug): calls the iterative function, not itself
return n if n < 10 else n % 10 + sum_of_digits(n // 10)

# After (fix): properly calls itself recursively
return n if n < 10 else n % 10 + sum_of_digits_recursion(n // 10)

Impact

• The function was not actually recursive as its name and documentation suggest
• The result was correct (it fell back to the iterative version), but the intent was wrong
• This is a correctness fix for the algorithm implementation

Checklist

• All doctests pass
• No behavioral change in output (the function still produces correct results)