chore: rename gcd to greatest_common_divisor (#6265)
chore: rename gcd to greatest_common_divisor (#6265)

As described in CONTRIBUTING.md

> Expand acronyms because gcd() is hard to understand but greatest_common_divisor() is not.

Co-authored-by: Dhruv Manilawala <482973f2a44b929137ace410c8397045079496c1@gmail.com>
TheAlgorithms/Python