Update calculator.py

[Anas-AbdelRaoof]

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[Anas-AbdelRaoof]

I added the function of Factorial. (by Anas-AbdelRaoof)

Notice: Factorial is the multiplying of numbers from 1 to n (as n is a _number) and it's symbol is !
Ex: fact(5) = 1 * 2 * 3 * 4 * 5 = 120