Create a function that will find all primes below a given number. Return the result as a list.
primes_below_num(5) ➞ [2, 3, 5]
primes_below_num(10) ➞ [2, 3, 5, 7]
primes_below_num(20) ➞ [2, 3, 5, 7, 11, 13, 17, 19]
If n is a prime, include it in the list.