added factorial in cpp snippet(code template for factorial in cpp).

[Thejas0604]

Dear Maintainers,

Just added code template for finding factorial in cpp. To check the functionality,

1. Add these lines above the code snippet.

   #include <iostream>
   using namespace std;

2. Add this function below the code snippet.

   int main()
   {
   int n = 5;
   cout << "Factorial of " << n << " is " << factorial(n) << endl;
   return 0;
   }

   Verified output:

Thank you.