input: Write a python function to calculate the sum of the reciprocals of the first n natural numbers.
  assert reciprocal_sum(100) == 5.187377517639621
  assert reciprocal_sum(1) == 1.0
  assert reciprocal_sum(10) == 2.9289682539682538
output: def reciprocal_sum(n):
      return sum(1 / i for i in range(1, n + 1))