input: Write a python function to calculate the sum of all numbers below a given number that are multiples of 3 or 5.
  assert sum_of_multiples(10) == 23
  assert sum_of_multiples(20) == 78
  assert sum_of_multiples(1000) == 233168
output: def sum_of_multiples(limit):
      return sum(x for x in range(limit) if x % 3 == 0 or x % 5 == 0)